FABRIC.md: item 4.5d -- arch_relax() tested against the PM Timer stall, disproven

Follow-up probing narrowed Finding 3's behavior (inconsistent stall point
run-to-run: sometimes reaches iters=1000/delta=951 before stopping,
sometimes never gets past iters=0 even after a 400-second bounded wait) but
didn't pin the mechanism.

Tested the strongest available hypothesis: single-threaded TCG scheduling
starvation from an -O2-tightened spin loop, based on a real precedent --
calibrate_apic_timer() (apic.c) already calls arch_relax() every iteration
of its own spin-wait; calibrate_tsc_with_pmtimer() never had it. Added the
same call, matching that precedent exactly. Result: no change, same exact
stall point on a fresh 60-second bounded wait. Reverted.

Stopping here per this document's own §25.0 rule 5 -- root-causing further
needs either deeper TCG/QEMU internals knowledge or a different diagnostic
approach (host-side instrumentation) than another guess-and-check pass.
Makefile.starkernel and timer.c both back to committed -O0 state, confirmed
booting clean.

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

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