v0.5.7: guard dumps block-header math + 16-word window; vt content verify; shadow marker

The klog -t 2 crash drops a 126-byte String (the vt verify klog line)
whose data ptr 0x40444970 points into the first-heap-block region whose
content is the firmware 0x00AAAAAA scrub — a stale pointer, not a real
allocation (back-pointer slot never written).  Upgrades so the next
real-hardware crash shows the full picture:
- guard: explicit candidate block bases (b=ptr-24 / b16=ptr-32) with
  their size/free fields, plus a 16-word window (mem..mem4) from
  ptr-0x30 instead of the old 8 words
- vt::debug_verify: content probe of the cells grid (must be all-BLANK
  at shell entry) — catches a stale in-heap cells pointer that passes
  the range checks
- gop: on-screen marker for shadow carved vs NO-SHADOW fallback
  (the scrub content at the crash ptr suggests the shadow was rejected
  and its block stayed free + firmware-scrubbed)

1 files changed, 30 insertions, 3 deletions

diff --git a/rsnix/src/vt.rs b/rsnix/src/vt.rs
index 2d8ece6..2906258 100644
--- a/rsnix/src/vt.rs
+++ b/rsnix/src/vt.rs
@@ -172,13 +172,40 @@ pub fn debug_verify() {
                 p >= hs && p + bytes <= he
             })
             .unwrap_or(false);
-        if c.len() != total || r.len() != total || !heap_ok {
+        // 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.
+        let mut content_ok = true;
+        if heap_ok && c.len() == total && total > 0 {
+            let mut i = 0usize;
+            // Sample the first cell, the last cell, and a few interior
+            // cells — reading the whole grid here would be ~60 KiB of
+            // checks at every boot for no real benefit.
+            let probes = [0usize, total / 3, (2 * total) / 3, total - 1];
+            for &p in probes.iter() {
+                if i >= probes.len() {
+                    break;
+                }
+                i += 1;
+                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 {
+                    content_ok = false;
+                    break;
+                }
+            }
+        }
+        if c.len() != total || r.len() != total || !heap_ok || !content_ok {
             msg("verify: FAILED — cells/rendered not sane at shell entry");
             crate::gop::boot_marker(&alloc::format!(
-                "vt: cells {:#x} {}/{} FAILED",
+                "vt: cells {:#x} {}/{} {}",
                 c.as_ptr() as usize,
                 c.len(),
-                total
+                total,
+                if content_ok { "range" } else { "CONTENT-GARBAGE" }
             ));
         } else {
             msg("verify: ok");