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>

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