kernel: reject late reaped-process syscalls

[brandonpayton]

Summary

• Return ESRCH when a channel syscall arrives for a PID already removed from the process table.
• Prevent late browser worker messages from reaching syscall handlers that assume current_pid still resolves.
• Add focused host regression coverage for late channel syscalls after reap.

Verification

• cargo test -p kandelo --target aarch64-apple-darwin --lib
• cd host && npx vitest run
• scripts/run-libc-tests.sh
• scripts/run-posix-tests.sh
• bash scripts/check-abi-version.sh
• Focused browser walcrash3 post-fix run timed out without the old RuntimeError: unreachable / handleFcntlLock signature; not claimed as a SQLite pass.