ARM64 parity plan updates + sigaltstack analysis

[ryanbreen]

Summary

• expand ARM64 parity plan with a concrete AMD64 vs ARM64 gap matrix
• document AMD64 sigaltstack failure analysis and likely root cause
• extend ARM64 HHDM map coverage and use HHDM for raw UART access
• preserve original user stack pointer for SA_ONSTACK delivery (interrupt path)

Notes

• syscall-return signal delivery still needs the same saved_rsp fix in kernel/src/syscall/handler.rs (Tier-1 file; requires explicit approval)

Testing

• not run (per instructions)