Fail fast on decompiler server startup errors

[subwire]

Summary

• capture detached decompiler-server stdout/stderr in a per-server log beside its socket
• monitor the spawned child while waiting for registry registration
• fail immediately when the child exits, including its status, log path, and final 8 KiB of output
• include the same bounded diagnostic tail on startup timeout
• add decompiler load --timeout SECONDS (default remains 300 seconds)
• return log_path when a new server starts successfully
• document the behavior in the CLI guide and bundled agent skill

Motivation

decompiler load previously sent detached child output to /dev/null and only watched the registry. A missing dependency, license problem, socket failure, or backend crash therefore looked identical to slow analysis and consumed the full five-minute timeout.

We reproduced this both during an ENOWARS workload replay and locally. In the local regression case, angr initialized but the server could not bind its socket. Before this change, load waited 300 seconds and reported only a generic timeout. With this change, it failed in 2.9 seconds and surfaced the exact child error:

Decompiler server ... exited with status 1 before registering.
Server log: .../declib_server_.../server.log
Server log tail:
...
Failed to start server: [Errno 1] Operation not permitted

The configurable timeout also lets automation choose between a strict task budget and unusually slow initial Ghidra/IDA analysis.

Testing

• lifecycle helper tests: early child exit, bounded log tail, timeout reporting, and parser contract — 5 passed
• tests/test_artifacts.py plus backend-independent test_decompiler_cli.py cases — 28 passed, 209 skipped, 1 pre-existing environment-sensitive test deselected
• documented core suite (tests/test_artifacts.py tests/test_cli.py) — 11 passed
• end-to-end failure regression — returned the child error and log in 2.9 seconds instead of 300 seconds
• end-to-end success regression — temporary angr server started in 2.2 seconds, JSON included an existing log_path, and the server stopped cleanly

The decompiler CI job now includes test_decompiler_cli.py, so the existing backend matrix also verifies that newly started IDA, Ghidra, Binary Ninja, and angr servers return a real log path where available.

[mahaloz]

LGTM!