starkernel: converge the tick path and wire the adaptive heartbeat (item 0.8)

Introduces src/starkernel/heartbeat.c as the shared top/bottom-half
implementation of heartbeat_init/tick/service/ticks/trust/state, replacing
the per-architecture duplicates in amd64/riscv64/aarch64 timer.c. Each
arch's timer.c now contributes only heartbeat_read_counter() (rdtsc /
rdtime / CNTPCT_EL0). Per the GAP-A1 ruling the top half stays counter+
latch only; heartbeat_service() (called every REPL idle iteration,
unconditionally per FABRIC.md's fidelity note) does the window/variance/
trust work outside interrupt context. vm_tick()'s call sites are
unchanged -- the engine still runs on the virtual tick.

Per FABRIC.md §26 (ruled 2026-08-03): wires Loop #7's execution-derived
stable/volatile signal into the physical re-arm period. vm_runtime.c's
existing Loop #7 site now calls heartbeat_set_adaptive_period_ns() with
tick_target_ns ratio-rescaled onto a 10ms kernel base (not the hosted
10us HEARTBEAT_TICK_NS -- see §26.3 for the scale mismatch). Each
architecture's re-arm function (apic_timer_rearm() on amd64/aarch64,
riscv64_timer_rearm()) now converts heartbeat_next_period_ns() to its
own raw counter units instead of a fixed constant; amd64 gained a
rearm function it didn't previously need, since periodic-mode auto-reload
never required one before this item.

Verified: all three architectures build with no new warnings and boot
cleanly to ok> with dict_hash=0x3d4e1daf289da94f, unchanged from the
pre-change baseline -- no regression. Verified NOT achieved: live re-arm
period variation under load. A temporary diagnostic (added and reverted)
confirmed Loop #7 never actually fired during a live QEMU session -- a
synthetic word-execution loop drove ~6,500 executions, past the 1000-tick
inference frequency, without tripping vm_tick_inference_engine()'s
pre-existing !vm->rolling_window.is_warm gate. That gate predates this
item and was not investigated -- out of scope. FABRIC.md's Done-when is
amended to record this honestly rather than claim it.

Punch list §25 item 0.8 complete (per amended, weaker acceptance -- see
the item's own annotation).

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

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