FABRIC-3.md: amd64 boot-path code audit pass (§III item 6)

Reviewed src/starkernel/boot/uefi_loader.c and every file in arch/amd64/
(arch.c, apic.c, ioapic.c, interrupts.c, i8042.c, timer.c) for anything that
assumes SER5-specific hardware rather than standard UEFI/ACPI. Report only,
per this project's "identify, don't fix unless asked" rule -- no source
files changed.

Verified clean: uefi_loader.c's COM1 presence probe, its two-pass ACPI
2.0-then-1.0 GUID preference, the GetMemoryMap/ExitBootServices golden
path, and GOP BltOnly degradation. apic.c/ioapic.c derive LAPIC mode,
APIC timer frequency, and I/O APIC base/GSI at runtime, not from hardcoded
constants.

Four findings recorded (not fixed):
1. HPET base (timer.c) hardcoded at 0xFED00000, never cross-checked against
   the ACPI "HPET" table -- the same bug class this file's own comment
   already documents and fixed for the PM_TMR port.
2. LAPIC base always 0xFEE00000; apic_init()'s own doc comment flags MADT
   relocation as unimplemented. Narrower than that suggests: ioapic.c's
   parser already reads MadtHeader.local_apic_address, just never plumbs
   it to apic.c. MADT type 5 (Local APIC Address Override) isn't parsed
   at all.
3. i8042 PS/2 controller (i8042.c) and the legacy 8259 PIC (interrupts.c)
   are poked unconditionally with no ACPI FADT presence check, unlike this
   same file's own raw_serial_init() COM1 probe.
4. arch_cold_reset() (confirmed live via BYE, mama_forth_words.c:1437)
   pulses the i8042 reset line instead of using the FADT's standards-defined
   RESET_REG/RESET_VALUE, which doesn't depend on i8042 existing at all.

None are certain to bite on the real SER5 -- the conventions assumed hold
on the large majority of PC-compatible x86_64 systems -- but the decision
in FABRIC-3.md §III requires arguing genericity from standards compliance,
not from "it booted," and these are the concrete gaps.

Doc-only change. Three-arch QEMU acceptance (amd64/aarch64/riscv64, in
order) run to confirm non-regression only.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019YcT3H2PQeyujrzjqS3Var

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