Resolve corpus compiler paths before changing the working directory #808

Problem

An explicit relative compiler path is validated in the caller directory but returned unchanged. Compilation later runs with cwd=ROOT; a valid ./fake-wavec becomes a bare fake-wavec executable name and cannot be launched. A compiler with that name on PATH could also be selected instead.

Starting points: tools/check_wave_corpus.py.

Reproduction

Executed on Linux amd64 with Python 3.14.7. The affected files at local ade878134519b0f957ca28a23c6f9cd98e43dff3 are byte-identical to canonical master 286dad9c8fcb0616f7c719c72cbf453a7d9925fd.

Run from the repository root:

python3 -B - <<'PYREPRO'
import os, tempfile
from pathlib import Path
from unittest.mock import patch
from tools import check_wave_corpus as c
with tempfile.TemporaryDirectory() as tmp:
    p = Path(tmp) / 'fake-wavec'
    p.write_text('#!/bin/sh\nexit 0\n')
    p.chmod(0o700)
    os.chdir(tmp)
    with patch.object(c, 'corpus_files', return_value=[c.ROOT / 'examples/std/net_tcp.wave']):
        c.main(['--wavec', './fake-wavec'])
PYREPRO

Observed: FileNotFoundError: [Errno 2] No such file or directory: fake-wavec. The provided executable exists and is executable.

Scope

Easy: make successfully resolved executable paths stable across the child working-directory change, including the WAVEC override. Keep explicit-selection precedence.

Completion criteria

• A caller-relative executable runs from outside the repository.
• Explicit paths cannot silently fall back to a same-named PATH executable.
• Cover relative paths containing spaces and WAVEC selection.