Fail corpus validation when no Wave sources are discovered

[LunaStev]

Problem

The corpus gate returns success when source discovery yields no files. An incomplete checkout or moved/deleted source tree can therefore produce a green validation run that checked nothing.

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.

python3 -B - <<'PYREPRO'
import sys
from pathlib import Path
from unittest.mock import patch
from tools import check_wave_corpus as c
with patch.object(c, 'corpus_files', return_value=[]), \
     patch.object(c, 'resolve_wavec', return_value=Path(sys.executable)):
    print('exit:', c.main([]))
PYREPRO

Observed: Corpus result: 0 passed, 0 failed and exit: 0. This isolates discovery; no compiler process is started.

Scope

Easy: reject an empty maintained corpus. When --run-std-examples is requested, also reject an empty std-example selection. This adds failure behavior beyond the general test-coverage tracking in #354.

Completion criteria

• An empty corpus returns nonzero with a clear diagnostic.
• Requested example execution cannot pass with zero selected examples.
• A nonempty passing selection still returns zero, and an empty selection launches no compiler.