klog: kernel log ring + /dev/klog + klog shell command

- rsnix/src/klog.rs: RAM ring of the last 128 serial log lines (128
  chars each), fed by serial_println! so every boot/low-level message
  is captured for free.  Sequence numbers per line, wrap/drop
  accounting; the ring never allocates (const-constructible static).
- /dev/klog char device; shell 'klog' is a thin alias for cat
  /dev/klog with the same filters/redirection, so 'klog -t 5',
  'klog -g fat' and 'klog WRITING /disk/.../boot.log' all work.
  klog rejects positional args (usage) and input keywords.
- Shell user-facing output (banner, prompt, term_print/term_println,
  pipeline sink, halt/poweroff) now writes to serial directly instead
  of through serial_println!, keeping echo/cat output out of the boot
  log (a streaming cat /dev/mem would otherwise wrap the ring).
- serial_println! macro hooks klog::append_fmt via format_args! (no
  alloc at call sites); klog::dump formats the log for the device.

Verified in QEMU: 28 boot lines captured (seq 0..27), shell output
absent from the ring, klog WRITING persists a boot log (mtype-checked),
wrap verified with a temporary 140-line boot loop (128 kept / 40
dropped, seq 40..167, oldest-live correct; loop reverted); full
test-disk round trip still fsck-clean.

1 files changed, 10 insertions, 1 deletions

diff --git a/rsnix/src/serial.rs b/rsnix/src/serial.rs
index 03fa570..484a114 100644
--- a/rsnix/src/serial.rs
+++ b/rsnix/src/serial.rs
@@ -91,12 +91,21 @@ macro_rules! serial_print {
     }};
 }
 
+/// Prints a line to serial and appends it to the kernel log ring
+/// (klog — see /dev/klog).  Every kernel-side message funnels through
+/// here, so the ring captures the whole boot sequence for free.
+/// User-facing shell output must NOT use this macro (shell output would
+/// pollute the boot log); it writes to the serial port directly.
 #[macro_export]
 macro_rules! serial_println {
-    () => { $crate::serial::write_str("\r\n"); };
+    () => {
+        $crate::serial::write_str("\r\n");
+        $crate::klog::append_fmt(core::format_args!(""));
+    };
     ($($arg:tt)*) => {{
         $crate::serial_print!($($arg)*);
         $crate::serial::write_str("\r\n");
+        $crate::klog::append_fmt(core::format_args!($($arg)*));
     }};
 }