FABRIC.md: item 4.5d -- fortification and putc/getc fixed, PM Timer stall now blocks

Finding 2 (putc/getc) fixed via shim.c backfill (previous commit). With
both Finding 1 (-U_FORTIFY_SOURCE) and Finding 2 fixed, amd64 links clean
at -O2 -- rigorously confirmed zero new warnings by diffing normalized
warning text between -O0 and -O2 builds (empty diff, ~3040 pre-existing
warnings in vendored code unchanged).

New Finding 3, not fixed, now blocking: amd64 boot stalls for minutes
inside calibrate_tsc_with_pmtimer() (timer.c:560-589) at -O2 -- a
mainline busy-wait loop reading the ACPI PM Timer via genuinely-volatile
inl() until 1000 real ticks elapse, bounded by a 5M-iteration timeout. A
one-shot diagnostic probe (reverted after capture) caught exactly one
checkpoint in 55 seconds of observation, then nothing -- not yet
root-caused whether this is the 5M-iteration timeout itself taking several
minutes to exhaust under -O2, or a genuine non-terminating condition.

Makefile.starkernel reverted to -O0 again; nothing broken landed in
history. Routine artifacts from this session's boot attempts included.

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

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