The critical fix: local_apic.rs id() now returns (read(0x20) >> 24) for xAPIC mode instead of the raw register value. This was causing wrong APIC IDs on Intel, leading to misrouted IPIs, missed TLB shootdowns, and the page fault during context switch at switch.rs:317.
Also adds:
- Named ICR constants (ICR_INIT_ASSERT, ICR_STARTUP) with bit-layout comments
- Comment documenting x2APIC timeout limitation (cpu_id allocated before timeout check)
- All existing changes preserved (x2APIC MADT type 9, cpuid split, memory alignment, RSDP checksum, ICR pending wait, MADT entry length guard)
Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)
Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>
vasilito
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