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.

system/kernel/kernel.zig

@@ -5,6 +5,7 @@ const parameters = @import("parameters");
 const architecture = @import("architecture");
 const console = @import("console.zig");
 const log = @import("log.zig");
+const wall_clock = @import("wall-clock.zig");
 const pmm = @import("pmm.zig");
 const heap = @import("heap.zig");
 const scheduler = @import("scheduler.zig");
@@ -285,6 +286,10 @@ fn kmain(boot_information: *const BootInformation) noreturn {
     if (!architecture.clockSynchronized())
         log.write("/system/kernel: WARNING: per-core TSCs are not synchronized; monotonic clock moved off the TSC\n");
 
+    // Anchor wall-clock time: read the RTC once, now the monotonic clock is final.
+    wall_clock.init();
+    log.print("/system/kernel: wall clock {d} (Unix epoch seconds, UTC, from the RTC)\n", .{wall_clock.nowSeconds()});
+
     // In a test build (`zig build -Dtest-case=<name>`), run that case and stop.
     // Normal builds fall through to the idle halt.
     if (build_options.test_case) |case| {