feat(kernel): dispatch AArch64 getpid

[kernalix7]

Summary

Adds the first live Linux AArch64 syscall path by routing getpid(2) through the existing EL0 SVC smoke test.

Changes

• normalize Linux AArch64 syscall 172 to the shared internal SYS_GETPID dispatcher
• forward saved x8 and x0..x5 through the lower-EL trap frame and write the result back to user x0
• verify result writeback in EL0 before emitting the new QEMU boot marker
• preserve the existing stack and SIMD/FP SVC syscalls 6/8: ftruncate/waitid/fchmod-fchown/fdatasync + pread/pwrite/readv/writev #7/syscalls 7/8: *at family + statfs/sysinfo/getrandom + advisory + uname/time/creds (Workflow) #8 smoke contracts
• update architecture-neutral syscall and AArch64 status documentation

Related Issues

N/A

Verification

• cargo fmt --all -- --check
• cargo clippy --workspace -- -D warnings
• cargo build --workspace
• bash scripts/run-tests.sh (4,849 passed)
• strict Miri on the four AArch64 ABI adapter tests
• AArch64 and RISC-V kernel cross-builds
• bash scripts/run-qemu-test.sh (x86_64, AArch64, and RISC-V passed)

Checklist

• cargo fmt --all -- --check - zero errors
• cargo clippy --workspace -- -D warnings - zero warnings
• cargo build --workspace - succeeds
• Public APIs are documented with /// comments
• unsafe blocks have // SAFETY: comments
• License header present in new files
• Relevant architecture and runner documentation updated
• No hardcoded paths, credentials, or personal info

Screenshots

N/A