v0.5.7: fix vt verify content check — expect BLANK (0x70020), not zero

The pre-verify 0x70020 'mystery' was a debug artifact, not corruption:
BLANK = {ch:32, fg:7, bg:0} = raw 0x70020, and vec![BLANK; n] fills both
grids with it at init.  The old check compared cells against 0 and its
probe loop break-ed on the first non-zero cell, so probes 1-3 stayed at
their zero-initialized slots — c=[0x70020,0,0,0] was bogus on every
healthy boot.  A full-grid dump proves QEMU grids are 100% BLANK at
shell entry (nz=3700 blank=3700).

Fix: probes compare against blank_raw (0x70020), record all four values
(no early break), probe both cells and rendered, and add a full-grid
nz/blank summary line.  The OptiPlex firmware 0x00AAAAAA scrub remains
cleanly distinguishable (scrub != blank).

Markers: the shell banner occupies grid rows 0-1 and the prompt row 2
(proven by screendump), and a stale crash.log replays at rows 2+, so the
shadow and vt markers moved to the bottom of the screen via the new
gop::text_rows()/bottom_row(off) helpers instead of rows 1-2.

QEMU verified: verify: ok every boot; test-msg + test-fault PASS; debug
and release builds 0 warnings.

2 files changed, 140 insertions, 36 deletions

diff --git a/rsnix/src/gop.rs b/rsnix/src/gop.rs
index 8e804e5..012a9d0 100644
--- a/rsnix/src/gop.rs
+++ b/rsnix/src/gop.rs
@@ -293,13 +293,16 @@ pub fn init(gop: &mut GraphicsOutputProtocol) {
                         .map(|c| core::str::from_utf8(c).unwrap_or("?"))
                         .collect();
                     dump_lines(&lines);
-                    crate::gop::boot_marker("gop: NO-SHADOW (fallback mode)");
+                    crate::gop::boot_marker_at("gop: NO-SHADOW (fallback)", bottom_row(0));
                 } else {
-                    crate::gop::boot_marker(&alloc::format!(
-                        "gop: shadow {:#x} {:.2} MiB",
-                        SHADOW as usize,
-                        swords as f64 * 4.0 / 1048576.0
-                    ));
+                    crate::gop::boot_marker_at(
+                        &alloc::format!(
+                            "gop: shadow {:#x} {:.2} MiB",
+                            SHADOW as usize,
+                            swords as f64 * 4.0 / 1048576.0
+                        ),
+                        bottom_row(0),
+                    );
                 }
             }
         }
@@ -313,6 +316,16 @@ pub fn init(gop: &mut GraphicsOutputProtocol) {
 /// direct-display glyph path (the shadow may not exist yet in early
 /// boot); the shell banner overwrites row 0 at first paint.
 pub fn boot_marker(msg: &str) {
+    boot_marker_at(msg, 0);
+}
+
+/// boot_marker at an explicit text row.  The shell's first paint
+/// occupies grid rows 0 (motd) and 1 (=== banner) and the prompt sits
+/// at row 2 — markers whose *values* matter after boot (shadow state,
+/// vt grid pointers) are anchored to the bottom of the screen via
+/// `bottom_row`, which also survives a stale crash.log replay (the
+/// fault band is ~20 lines starting at grid row 2).
+pub fn boot_marker_at(msg: &str, row: usize) {
     // Record the stage so a later panic band can show how far boot got.
     unsafe {
         let n = msg.len().min(STAGE_MAX);
@@ -323,24 +336,39 @@ pub fn boot_marker(msg: &str) {
     let Some(fb) = fb().as_ref() else {
         return;
     };
+    if row >= fb.height / 16 {
+        return;
+    }
     let cols = fb.width / 8;
     let white = (0xFFu8, 0xFFu8, 0xFFu8);
     let black = (0x00u8, 0x00u8, 0x00u8);
     let mut c = 0usize;
     for &b in msg.as_bytes().iter().take(cols) {
         if b < 0x80 {
-            blit_glyph(c, 0, b as u16, white, black);
+            blit_glyph(c, row, b as u16, white, black);
             c += 1;
         }
     }
     // Blank the rest of the row so a shorter message can't leave stale
     // glyphs behind.
     while c < cols {
-        blit_glyph(c, 0, b' ' as u16, white, black);
+        blit_glyph(c, row, b' ' as u16, white, black);
         c += 1;
     }
 }
 
+/// Text-row count of the active framebuffer (each row is 16 px tall).
+pub fn text_rows() -> usize {
+    fb().as_ref().map(|f| f.height / 16).unwrap_or(0)
+}
+
+/// Row index of the `off`-th text row from the bottom (0 = last row).
+/// Bottom-anchored markers survive the shell banner + prompt at the top
+/// and a stale crash.log replay below it.
+pub fn bottom_row(off: usize) -> usize {
+    text_rows().saturating_sub(1 + off)
+}
+
 // ---------------------------------------------------------------------------
 // Public text API — the text surface now lives in `crate::vt`; these wrappers
 // keep callers (shell, macros) unchanged.
diff --git a/rsnix/src/vt.rs b/rsnix/src/vt.rs
index 2906258..6e1f12f 100644
--- a/rsnix/src/vt.rs
+++ b/rsnix/src/vt.rs
@@ -173,48 +173,124 @@ pub fn debug_verify() {
             })
             .unwrap_or(false);
         // Content check: at shell entry nothing has been printed yet, so
-        // every cell must still be BLANK (all-zero).  A stale cells data
-        // pointer that happens to be in-heap still passes the range
-        // checks (len/cap look fine) but its "cells" read back as the
-        // underlying memory's leftover fill — the OptiPlex shows the
-        // firmware 0x00AAAAAA scrub there.  This catches that.
+        // every cell must still hold the grid's init fill.  NOTE: that
+        // fill is BLANK (space + fg 7 + bg 0 = raw 0x70020), NOT zero —
+        // `vec![BLANK; n]` fills the grid with BLANK, and all boot
+        // output to this point is serial-only (boot markers are a
+        // direct-FB blit, never the vt grid).  The v0.5.7 check
+        // compared against 0 and therefore tripped on every healthy
+        // boot (the "0x70020 at grid(0,0)" mystery — the grid's own
+        // init value, with the probe loop's early `break` leaving the
+        // other three probe slots as zero-initialized).  A stale cells
+        // data pointer that happens to be in-heap still passes the
+        // range checks (len/cap look fine) but its "cells" read back as
+        // the underlying memory's leftover fill — the OptiPlex shows
+        // the firmware 0x00AAAAAA scrub there.  BLANK (0x70020) vs
+        // scrub (0x00AAAAAA) is cleanly distinguishable.
+        let blank_raw = BLANK.ch as u32 | ((BLANK.fg as u32) << 16) | ((BLANK.bg as u32) << 24);
         let mut content_ok = true;
+        let mut rcontent_ok = true;
+        let mut c_probes_raw: [u32; 4] = [0; 4];
+        let mut r_probes_raw: [u32; 4] = [0; 4];
         if heap_ok && c.len() == total && total > 0 {
-            let mut i = 0usize;
-            // Sample the first cell, the last cell, and a few interior
+            // Sample the first cell, the last cell, and two interior
             // cells — reading the whole grid here would be ~60 KiB of
-            // checks at every boot for no real benefit.
+            // checks at every boot for no real benefit.  All four
+            // probes are recorded unconditionally (no early break) so
+            // a serial log shows every value.
             let probes = [0usize, total / 3, (2 * total) / 3, total - 1];
-            for &p in probes.iter() {
-                if i >= probes.len() {
-                    break;
-                }
-                i += 1;
+            for (pi, &p) in probes.iter().enumerate() {
                 let cell = *c.as_ptr().add(p);
                 let raw = cell.ch as u32 | ((cell.fg as u32) << 16) | ((cell.bg as u32) << 24);
-                if raw != 0 {
+                c_probes_raw[pi] = raw;
+                if raw != blank_raw {
                     content_ok = false;
-                    break;
+                }
+            }
+            // Same probe on the rendered grid: it is a *separate*
+            // allocation, so its content discriminates "only the cells
+            // VecData's ptr field is stale" (rendered ok) from "both
+            // grids' memory was clobbered / both VecData ptrs are
+            // stale" (rendered garbage too).
+            for (pi, &p) in probes.iter().enumerate() {
+                let cell = *r.as_ptr().add(p);
+                let raw = cell.ch as u32 | ((cell.fg as u32) << 16) | ((cell.bg as u32) << 24);
+                r_probes_raw[pi] = raw;
+                if raw != blank_raw {
+                    rcontent_ok = false;
+                }
+            }
+        }
+        // Full-grid summary (cheap: a few thousand reads, serial-only).
+        // Definitive health signal on QEMU: healthy = every cell is
+        // BLANK (nz == blanks == total); the probes above are just the
+        // tripwire, this catches a partial clobber that the four
+        // samples happen to miss.
+        let (mut c_nz, mut c_blank) = (0usize, 0usize);
+        let (mut r_nz, mut r_blank) = (0usize, 0usize);
+        if heap_ok && c.len() == total && r.len() == total && total > 0 {
+            for i in 0..total {
+                let cell = *c.as_ptr().add(i);
+                let raw = cell.ch as u32 | ((cell.fg as u32) << 16) | ((cell.bg as u32) << 24);
+                if raw != 0 {
+                    c_nz += 1;
+                }
+                if raw == blank_raw {
+                    c_blank += 1;
+                }
+                let cell = *r.as_ptr().add(i);
+                let raw = cell.ch as u32 | ((cell.fg as u32) << 16) | ((cell.bg as u32) << 24);
+                if raw != 0 {
+                    r_nz += 1;
+                }
+                if raw == blank_raw {
+                    r_blank += 1;
                 }
             }
         }
-        if c.len() != total || r.len() != total || !heap_ok || !content_ok {
+        msg(&alloc::format!(
+            "verify: grids c nz={} blank={} r nz={} blank={} (of {})",
+            c_nz, c_blank, r_nz, r_blank, total
+        ));
+        if c.len() != total || r.len() != total || !heap_ok || !content_ok || !rcontent_ok {
             msg("verify: FAILED — cells/rendered not sane at shell entry");
-            crate::gop::boot_marker(&alloc::format!(
-                "vt: cells {:#x} {}/{} {}",
-                c.as_ptr() as usize,
-                c.len(),
-                total,
-                if content_ok { "range" } else { "CONTENT-GARBAGE" }
+            msg(&alloc::format!(
+                "verify: probes c=[{:#x},{:#x},{:#x},{:#x}] r=[{:#x},{:#x},{:#x},{:#x}]",
+                c_probes_raw[0],
+                c_probes_raw[1],
+                c_probes_raw[2],
+                c_probes_raw[3],
+                r_probes_raw[0],
+                r_probes_raw[1],
+                r_probes_raw[2],
+                r_probes_raw[3]
             ));
+            // Bottom-anchored: row 0-1 get overwritten by the shell
+            // banner, row 2 by the prompt (and a stale crash.log
+            // replays at rows 2+), so this marker must live at the very
+            // bottom to stay readable until the user can copy it.
+            crate::gop::boot_marker_at(
+                &alloc::format!(
+                    "vt: c {:#x} cap={:#x} {} | r {:#x} {}",
+                    c.as_ptr() as usize,
+                    c.capacity(),
+                    if content_ok { "ok" } else { "GARBAGE" },
+                    r.as_ptr() as usize,
+                    if rcontent_ok { "ok" } else { "GARBAGE" }
+                ),
+                crate::gop::bottom_row(1),
+            );
         } else {
             msg("verify: ok");
-            crate::gop::boot_marker(&alloc::format!(
-                "vt: cells {:#x} {}/{} ok",
-                c.as_ptr() as usize,
-                c.len(),
-                total
-            ));
+            crate::gop::boot_marker_at(
+                &alloc::format!(
+                    "vt: c {:#x} cap={:#x} ok | r {:#x} ok",
+                    c.as_ptr() as usize,
+                    c.capacity(),
+                    r.as_ptr() as usize
+                ),
+                crate::gop::bottom_row(1),
+            );
         }
     }
 }