The catch-all "dispatch threw" log omitted the request command, so a
handler exception couldn't be correlated to what caused it (PTY/IPC
audit item #26). Track the resolved cmd across the request handler and
include it in the error log.
The audit's other two items were already satisfied: errno/signal magic
numbers are centralized in PosixErrno + ffi/libc.dart, and the IPC
server already logs through the kernel Logger (no stderr.writeln left).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>