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, 8 insertions, 0 deletions

diff --git a/rsnix/src/fs.rs b/rsnix/src/fs.rs
index a71e51a..063560a 100644
--- a/rsnix/src/fs.rs
+++ b/rsnix/src/fs.rs
@@ -41,6 +41,7 @@ enum CharDevice {
     Pci,
     Version,
     Meminfo,
+    Klog,
     Null,
     Zero,
     Console,
@@ -141,6 +142,12 @@ pub fn init() -> (usize, usize) {
         NodeData::Char(CharDevice::Meminfo),
     );
     add(
+        "klog",
+        Some(dev),
+        FileType::Char,
+        NodeData::Char(CharDevice::Klog),
+    );
+    add(
         "null",
         Some(dev),
         FileType::Char,
@@ -650,6 +657,7 @@ fn dev_text(dev: CharDevice, _pos: u64) -> String {
         CharDevice::Pci => pci_listing(),
         CharDevice::Version => version_text(),
         CharDevice::Meminfo => meminfo_text(),
+        CharDevice::Klog => crate::klog::dump(),
         CharDevice::Null | CharDevice::Console | CharDevice::Serial => String::new(),
         CharDevice::Zero => String::new(),
     }