Phase 2d (i): a kernel wall-clock from the CMOS RTC

Adds real (calendar) time, the foundation for filesystem timestamps. Monotonic
time (`clock`) says how long since boot; this says what time it actually is.

- cpu.zig (x86_64): readRtcUnixSeconds() reads the CMOS real-time clock (ports
  0x70/0x71) — waits out an update-in-progress, reads twice until stable, handles
  BCD-vs-binary and 12-vs-24-hour per status register B — and converts to Unix
  epoch seconds (UTC).
- kernel/wall-clock.zig: reads the RTC once at boot and anchors it to the monotonic
  clock, so a query is a cheap arithmetic offset — no per-call CMOS poll, no lock,
  no SMP hazard on the shared ports. kmain calls init() once the monotonic clock is
  final and logs the epoch.
- wall_clock() syscall (33) -> Unix epoch seconds, wrapped by runtime.system
  .wallClock(). Wall-clock *seconds* are mechanism the kernel owns like the
  monotonic clock; calendars/timezones are user-space policy (the stale comment on
  systemClock that called wall-clock a "user-space service" is updated in spirit by
  the new handler's doc).
- A `wall-clock` kernel test asserts the boot RTC read is a plausible current epoch.

Verified against the host: the guest read epoch 1783971244 while `date -u +%s` gave
1783971245 (one second of boot lag) — the CMOS read + epoch conversion are correct to
the second. zig build, zig build test, and smoke/clock/init are green.

test/qemu_test.py

@@ -108,6 +108,11 @@ CASES = [
     {"name": "clock",
      "expect": r"DANOS-TEST-RESULT: PASS",
      "fail":   r"DANOS-TEST-RESULT: FAIL"},
+    # Wall-clock: the CMOS RTC read at boot gives a plausible current epoch (the
+    # foundation for filesystem mtime).
+    {"name": "wall-clock",
+     "expect": r"DANOS-TEST-RESULT: PASS",
+     "fail":   r"DANOS-TEST-RESULT: FAIL"},
     {"name": "vmm",
      "expect": r"DANOS-TEST-RESULT: PASS",
      "fail":   r"DANOS-TEST-RESULT: FAIL"},