rsnix v0.5.2: fault diagnostics + real-hardware scroll-crash hardening

The real-hardware scroll crash (white bands + instant freeze, QEMU-clean)
was invisible because the only trace was a single serial line on machines
with no serial console.  This makes every crash self-identifying and
hardens the real-hardware failure surfaces:

- Every CPU exception vector 0-31 now has an IDT entry (macro-generated
  stubs; 8/14 keep their IST stacks).  A fault in the shell prints a
  white-on-red band to the framebuffer via gop::dump_lines (raw fb
  writes bypassing the vt, fully clamped) plus a parseable serial line:
  '!!! <kind> vec=N err=… rip=… cr2=… rsp=… rflags=… task=N "name"',
  then halts — instead of converging to a cryptic double fault.
  Background-task faults abort the task as before.
- Boot log now reports the authoritative post-SetMode geometry:
  'gop: fb base=… size=… pitch=… WxH bpp=… bgr=…', so the next real
  machine's report identifies whether pitch*height exceeds the mapping.
- Framebuffer writes (clear, push_rect, shift_up_n bulk push, cursor
  overlay) are clamped to the firmware's framebuffer_size: a pitch-
  padded mode degrades to a truncated view instead of faulting past
  the mapping.
- The GOP framebuffer region is excluded from the page allocator (and
  subtracted from the heap's conventional-region pick) even when the
  firmware lists it as ConventionalMemory — kernel allocations can
  never overwrite the display again.  The GOP protocol is located and
  peeked before the memory map is captured so the exclusion is known
  at heap-init time.
- Shell gains a hidden 'fault' command (deliberate unmapped write) and
  'make test-fault' verifies the dump: vec=14, cr2=0x400000000000,
  task=0, halt — no triple fault.  All other suites re-pass.
- CONTEXT.md: fault-dump format documented for real-hardware reports.

1 files changed, 18 insertions, 10 deletions

diff --git a/rsdos/src/main.rs b/rsdos/src/main.rs
index 13923ea..6dbad5f 100644
--- a/rsdos/src/main.rs
+++ b/rsdos/src/main.rs
@@ -87,16 +87,10 @@ pub unsafe extern "efiapi" fn efi_main(image_handle: Handle, system_table: *mut
     rsnix::serial::init();
     rsnix::serial_println!("=== rsnix boot ===");
 
-    // Stash the memory map
-    let map_key = match rsnix::memory::capture_memory_map(bs) {
-        Ok(k) => k,
-        Err(e) => {
-            rsnix::serial_println!("mem: capture_memory_map failed 0x{:x}", e);
-            0
-        }
-    };
-
-    // Locate GOP (Graphics Output Protocol)
+    // Locate GOP (Graphics Output Protocol) *before* the memory map is
+    // captured: the heap/page allocators need to know the framebuffer
+    // region so they never hand out its pages (some real firmware lists
+    // it as conventional memory).
     let mut gop_interface: *mut core::ffi::c_void = core::ptr::null_mut();
     let gop_guid = EFI_GRAPHICS_OUTPUT_PROTOCOL_GUID;
     let gop_result = unsafe {
@@ -108,6 +102,20 @@ pub unsafe extern "efiapi" fn efi_main(image_handle: Handle, system_table: *mut
     };
     if gop_result == EFI_SUCCESS {
         let gop = unsafe { &mut *(gop_interface as *mut GraphicsOutputProtocol) };
+        rsnix::gop::peek_fb_region(gop);
+    }
+
+    // Stash the memory map
+    let map_key = match rsnix::memory::capture_memory_map(bs) {
+        Ok(k) => k,
+        Err(e) => {
+            rsnix::serial_println!("mem: capture_memory_map failed 0x{:x}", e);
+            0
+        }
+    };
+
+    if gop_result == EFI_SUCCESS {
+        let gop = unsafe { &mut *(gop_interface as *mut GraphicsOutputProtocol) };
         rsnix::serial_println!("gop: framebuffer found");
         rsnix::gop::init(gop);
     } else {