klog: [ssss.mmm] millisecond timestamps from the PIT

- interrupts::uptime_ms(): monotonic ms since boot — the 100 Hz PIT
  tick (10 ms) plus interpolation from the PIT's live 1.19 MHz
  down-counter (read twice around the tick load; a count jump means a
  tick fired between reads) for true sub-ms resolution.  TICKS widened
  AtomicU16 -> AtomicU64 (u16 wrapped after ~11 min).
- PIT_READY gates interpolation until interrupts::init configures the
  timer, so early-boot lines are deterministically [0000.000] instead
  of uninitialised-PIT noise; klog clamps timestamps monotonic.
- The CMOS RTC cannot provide milliseconds (1 Hz BCD device, seconds
  only) — millis come from the PIT.
- klog stores ms per line at append time; dump format is now
  '[0000.123] 000000  === rsnix boot ===' (user-specified format).

Verified in QEMU: first line [0000.000], Phase-2 lines show real
monotonic ms (fs: vfs ready at [0000.519], self-test [0000.524]),
per-dump monotonicity, boot.log persists timestamps, test-disk round
trip fsck-clean.

1 files changed, 12 insertions, 1 deletions

diff --git a/CONTEXT.md b/CONTEXT.md
index 86822d1..fd83245 100644
--- a/CONTEXT.md
+++ b/CONTEXT.md
@@ -259,7 +259,18 @@ Workspace of four crates, all built for the `x86_64-unknown-uefi` target:
   lines (128 chars each, ~16 KB), fed by `serial_println!` (one choke
   point — every boot/low-level message captured for free).  Sequence
   numbers per line + wrap/drop accounting; a dump header reports how
-  many lines were lost to wrap.  Exposed as the `/dev/klog` char
+  many lines were lost to wrap.  Each line carries a `[ssss.mmm]`
+  timestamp — monotonic ms since boot from `interrupts::uptime_ms()`:
+  the 100 Hz PIT tick (10 ms) plus interpolation from the PIT's live
+  1.19 MHz down-counter read twice around the tick load (a jump = a
+  tick fired between reads) for true sub-ms resolution; `TICKS` widened
+  to u64 so uptime never wraps.  **The CMOS RTC cannot provide
+  milliseconds** — it is a 1 Hz BCD device (seconds only) — so the
+  millis come from the PIT.  Early-boot lines are `[0000.000]`
+  (PIT not yet configured; `PIT_READY` gates interpolation), and
+  timestamps are clamped monotonic in the ring.
+
+  Exposed as the `/dev/klog` char
   device; the shell `klog` command is a thin alias for `cat
   /dev/klog` (same filters/redirection), so `klog -t 5`, `klog -g
   fat`, and `klog WRITING /disk/…/boot.log` (persists the boot log)