starkernel: TimeTrustState.ticks -- mark volatile, fixing the one hazard 4.5a found

Written directly in ISR context on all three architectures
(heartbeat_tick(), heartbeat.c:163) and read directly by mainline
(heartbeat_ticks(), including the busy-wait at kernel_main.c:880) without
being volatile -- worked by accident at -O0, would be a real bug once the
kernel builds with optimization (item 4.5). Every other field in this
struct is mainline-only (heartbeat_service()'s deferred window/variance/
trust processing), so only this one field needed the qualifier.

Punch list item 4.5b complete.
Three-arch acceptance boot clean at unchanged -O0 (no behavior change
intended yet -- this is prep for enabling optimization, not the switch
itself).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

include/starkernel/timer.h

@@ -87,7 +87,14 @@ typedef struct time_window {
  */
 typedef struct time_trust_state {
     /* Core counters */
-    uint64_t     ticks;            /* TIME-TICKS: monotonic heartbeat count */
+    volatile uint64_t ticks;       /* TIME-TICKS: monotonic heartbeat count --
+                                     * written directly in ISR context
+                                     * (heartbeat_tick(), all three archs) and
+                                     * read directly by mainline
+                                     * (heartbeat_ticks()); genuinely
+                                     * concurrent, unlike every other field in
+                                     * this struct (FABRIC.md item 4.5a/4.5b,
+                                     * 2026-08-11). */
     uint64_t     last_tsc;         /* TSC at last heartbeat */
     uint64_t     expected_delta;   /* Expected TSC ticks per heartbeat */