Fix HEARTBEAT-TICKS@: read heartbeat_ticks() (ISR hardware timer), not

vm->heartbeat.tick_count (FORTH-dispatch counter)

Captain Bob's law is unambiguous: the adaptive heartbeat is the one and
only clock, full stop. The first cut of this word read the wrong
counter under that name -- vm->heartbeat.tick_count is a colon-word-
dispatch counter gated at a fixed cadence (frozen during idle, blind to
per-dispatch CPU cost, see FABRIC-2.md Section Q). The real adaptive
heartbeat is heartbeat_ticks() in src/starkernel/heartbeat.c, driven
directly by the ISR-latched 100Hz hardware timer -- genuinely
time-based, confirmed advancing during idle wall-clock time on all
three architectures (amd64 4039->5510, aarch64 6126->7607, riscv64
2965->4466, each over ~15s idle). Kernel build only (__STARKERNEL__);
hosted build has no ISR timer and keeps the old fallback.

Three-arch QEMU acceptance: POST 1012/0/0 on each, word live-tested.

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

src/word_source/starforth_words.c

@@ -47,6 +47,7 @@
 #include "../../include/vm.h"
 #ifdef __STARKERNEL__
 #include "starkernel/vm/arena.h"
+#include "starkernel/timer.h"
 #endif
 #include "include/vocabulary_words.h"
 #include "../../include/version.h"
@@ -797,14 +798,23 @@ static void starforth_word_zuse_authenticate(VM *vm)
  * @brief Read-only accessor for the canonical heartbeat tick counter
  *
  * Stack effect: ( -- n )
- * Pushes vm->heartbeat.tick_count -- the one clock this project's timing
- * measurements are supposed to read, not host wall-clock. Read-only: no
- * corresponding store word exists or should exist.
+ * On the kernel build, pushes heartbeat_ticks() -- the ISR-driven hardware
+ * timer tick count (src/starkernel/heartbeat.c), the one real clock in this
+ * project. NOT vm->heartbeat.tick_count, which is a FORTH colon-word-dispatch
+ * counter gated at a fixed cadence -- frozen during idle time and blind to
+ * any cost that doesn't change control flow (see FABRIC-2.md Section Q).
+ * On the hosted build (no ISR timer), falls back to vm->heartbeat.tick_count
+ * since that's the only tick source available there.
+ * Read-only: no corresponding store word exists or should exist.
  * @param vm Pointer to the VM instance
  */
 static void starforth_word_heartbeat_ticks(VM* vm)
 {
+#ifdef __STARKERNEL__
+    vm_push(vm, (cell_t)heartbeat_ticks());
+#else
     vm_push(vm, (cell_t)vm->heartbeat.tick_count);
+#endif
 }
 
 /**