fix(tools): treat explicit wavec overrides as authoritative in corpus checker

[Visio-Vanitas]

Summary

Treat explicit --wavec arguments and non-empty WAVEC environment variables as authoritative in tools/check_wave_corpus.py, immediately raising FileNotFoundError if the specified binary cannot be found or resolved rather than silently falling back to local build discovery.

Motivation

Resolves #564. Previously, resolve_wavec combined explicit --wavec, WAVEC, and local build candidate paths into a single candidate list. If the user provided an invalid or misspelled --wavec path (or WAVEC env variable), the corpus checker fell back to local build paths, potentially validating a stale or unrelated binary.

Target and compatibility impact

None. Discovery fallback is retained when neither --wavec nor WAVEC is provided or when WAVEC is empty.

Validation

• Added unit tests in tools/test_check_wave_corpus.py covering:
  • Invalid explicit --wavec path fails immediately without falling back to existing build outputs.
  • Invalid explicit WAVEC environment variable fails immediately without selecting fallback.
  • Valid explicit --wavec path is selected.
  • Valid WAVEC environment variable is selected.
  • No override falls back to local build discovery.
  • Empty WAVEC environment variable falls back to discovery.
  • No override and missing binary raises FileNotFoundError.
• Validated test suites:
  • python3 -m unittest tools.test_case_manifest tools.test_test_contracts tools.test_process_tree tools.test_check_wave_corpus
  • python3 -m unittest discover -s tools -p "test_*.py"
  • python3 -m py_compile x.py tools/*.py

Checklist

• Commits include a DCO Signed-off-by line.
• Tests cover new behavior or the PR explains why no test is needed.
• User-facing changes include documentation or diagnostics updates.
• The change preserves the license boundary between the compiler and std/.

[LunaStev]

LGTM