v0.5.5: self-diagnosing boot — vt refuses 0x0 geometry, panic band shows last stage

The user's real-hardware boot showed 'PANIC: index out of bounds (len 0,
index 0) at vt.rs:146' — the vt grid was created empty because the
framebuffer geometry was 0x0 at vt::init (GOP missing or the active
mode reporting zero resolution).  The v0.5.4 blank screen was the same
panic, previously invisible (no screen dump in the panic handler).

- vt::init: rows/cols == 0 now halts with a visible white-on-red
  'FATAL: no framebuffer geometry — GOP missing or active mode is 0x0'
  band instead of building an empty grid that panics at the first
  cell write.
- boot_marker records the last boot stage; the panic band appends it
  as a 4th line so a crash shows exactly how far boot got.
- The gop+vt marker now carries the measured geometry ('rsnix: gop
  800x600 vt 100x37 ok') so a good boot is self-confirming on the
  screen.
- All 5 test suites pass; build/clippy clean.

1 files changed, 20 insertions, 0 deletions

diff --git a/rsnix/src/vt.rs b/rsnix/src/vt.rs
index 53821b7..4578f6c 100644
--- a/rsnix/src/vt.rs
+++ b/rsnix/src/vt.rs
@@ -96,6 +96,26 @@ static mut CUR: usize = 0;
 /// Allocate the cell grid.  Call once after `gop::init()`.
 pub fn init() {
     let (cols, rows) = gop::dims();
+    if rows == 0 || cols == 0 {
+        // No usable framebuffer geometry: GOP is missing entirely or the
+        // active mode reports 0x0.  Without a display there is nothing
+        // the shell can do, so stop with a visible explanation instead
+        // of building an empty grid and panicking at the first cell
+        // write (a blank screen on real hardware, which has no serial
+        // console).
+        let lines = [
+            "FATAL: no framebuffer geometry",
+            "GOP missing or active mode is 0x0",
+            "see the 'gop:' boot marker / serial log",
+        ];
+        crate::gop::dump_lines(&lines);
+        crate::serial_println!("FATAL: no framebuffer geometry (GOP missing or 0x0 mode)");
+        loop {
+            unsafe {
+                core::arch::asm!("cli; hlt");
+            }
+        }
+    }
     unsafe {
         ROWS = rows;
         COLS = cols;