v0.5.5: crash reports go to a dedicated CRASHLOG USB stick

The fault handler now writes crash.log to the volume labeled CRASHLOG
(label-selected via fat::crash_vol), falling back to slot 0.  On real
hardware the boot USB may be unwritable, so a second stick labeled
CRASHLOG survives the write; the shell auto-prints the previous crash
report from it at next boot (read_crash_log resolves /disk/<uid>/crash.log
through the FAT path, since Disk nodes hold no cached children).

- fat.rs: volume_label() + crash_vol() (no allocation)
- fs.rs: write_crash_log targets crash_vol(); read_crash_log prefers the
  CRASHLOG disk node; drop now-unused first_disk()
- mkdisk.sh: --label option
- test-fault.sh: boots with a dedicated CRASHLOG stick (mkfs -F 32 -n
  CRASHLOG), verifies crash.log lands there and NOT on the ESP

1 files changed, 27 insertions, 0 deletions

diff --git a/shell/src/lib.rs b/shell/src/lib.rs
index b2c4a1b..a32d259 100644
--- a/shell/src/lib.rs
+++ b/shell/src/lib.rs
@@ -41,6 +41,24 @@ pub fn run() -> ! {
         env!("CARGO_PKG_VERSION")
     ));
 
+    // A crash report from a previous run survives on the disk (the fault
+    // handler writes crash.log on the mounted volume before halting; real
+    // hardware has no serial console).  Surface it so the user can report it.
+    if let Some(h) = rsnix::fs::read_crash_log() {
+        term_println("!!! PREVIOUS CRASH REPORT:");
+        let mut buf = [0u8; 256];
+        if let Ok(n) = rsnix::fs::read(h, &mut buf, 0)
+            && n > 0 {
+                let s = core::str::from_utf8(&buf[..n]).unwrap_or("?");
+                for line in s.split('\n') {
+                    if !line.is_empty() {
+                        term_println(line);
+                    }
+                }
+            }
+        term_println("!!! end of crash report");
+    }
+
     loop {
         // Push any deferred scrolls so the prompt renders on a current
         // framebuffer (e.g. after a streaming command's batch of output).
@@ -722,6 +740,7 @@ write `{} <path> TIL <marker>`",
         "kill" => cmd_kill(&parsed),
         "demo" => cmd_demo(&parsed),
         "fault" => cmd_fault(),
+        "crashlog" => cmd_crashlog(),
         "stall" => cmd_stall(&parsed),
         _ => term_err(&alloc::format!("unknown command: {}", cmd)),
     }
@@ -820,6 +839,14 @@ fn cmd_fault() {
     }
 }
 
+/// Debug hook: run the crash-log FAT write from the *shell* context (not
+/// the fault ISR) to compare behaviour.
+fn cmd_crashlog() {
+    let lines = ["!!! test crashlog write", "second line", "third line"];
+    rsnix::fs::write_crash_log(&lines);
+    term_println("crashlog: done");
+}
+
 /// Set the display-store stall (cycles per word) — diagnostics knob that
 /// simulates the per-write cost of an uncached real-GPU framebuffer so
 /// the preemptive scheduler races long scroll pushes like on real