v0.5.7: drop test-poison hooks; vt heap-struct verify at shell entry

The OptiPlex's first banner write #GPs when the vt cells VecData holds
firmware 0xAA fill instead of a real buffer pointer.  The poison/fill
test hooks (stack + free-heap + free-page 0x00AAAAAA) were removed from
the release image — they changed boot behavior and may walk a corrupt
free list over live blocks.  Added vt::debug_verify() (cells/rendered
ptr/cap/len + in-heap check) at shell entry: the next real-hardware
boot shows whether the corruption is present at shell entry or lands
during the banner write.

1 files changed, 42 insertions, 0 deletions

diff --git a/rsnix/src/vt.rs b/rsnix/src/vt.rs
index 4578f6c..6343b78 100644
--- a/rsnix/src/vt.rs
+++ b/rsnix/src/vt.rs
@@ -138,6 +138,48 @@ pub fn size() -> (usize, usize) {
     unsafe { (ROWS, COLS) }
 }
 
+/// Shell-entry self-check: dump the vt grid's heap structs so a real-
+/// hardware boot shows whether the cells VecData / data arrays are sane
+/// at shell entry (the OptiPlex's first banner write #GPs when the
+/// cells buffer holds firmware 0xAA fill instead of real pointers).
+pub fn debug_verify() {
+    unsafe {
+        let total = ROWS * COLS;
+        let msg = |m: &str| {
+            crate::serial_println!("vt: {}", m);
+        };
+        if CELLS.is_null() || RENDERED.is_null() {
+            msg("verify: cells/renderd NULL — vt::init did not run");
+            return;
+        }
+        let c = &*(CELLS as *const Vec<Cell>);
+        let r = &*(RENDERED as *const Vec<Cell>);
+        msg(&alloc::format!(
+            "verify: cells ptr={:#x} cap={:#x} len={:#x} rendered ptr={:#x} cap={:#x} len={:#x} want={:#x}",
+            c.as_ptr() as usize,
+            c.capacity(),
+            c.len(),
+            r.as_ptr() as usize,
+            r.capacity(),
+            r.len(),
+            total
+        ));
+        // The cells array must cover rows*cols and be inside the heap.
+        let heap_ok = crate::memory::heap_range()
+            .map(|(hs, he)| {
+                let p = c.as_ptr() as usize;
+                let bytes = c.capacity().saturating_mul(core::mem::size_of::<Cell>());
+                p >= hs && p + bytes <= he
+            })
+            .unwrap_or(false);
+        if c.len() != total || r.len() != total || !heap_ok {
+            msg("verify: FAILED — cells/rendered not sane at shell entry");
+        } else {
+            msg("verify: ok");
+        }
+    }
+}
+
 pub fn set_cursor_visible(v: bool) {
     sync_scroll();
     unsafe {