Fix zero-fixture false-pass gate

[yangfei222666-9]

Summary

• Reject missing fixture directories in the False-Pass Gate self-test.
• Reject empty fixture directories so self_test cannot pass with cases=0.
• Narrow public wording to schema-level evidence-pointer and cannot_claim boundaries.

Verification

• python3 scripts/check_false_pass_gate.py --self-test examples/false_pass_gate/fixtures
• uv run --no-project --with pytest python -m pytest tests/test_false_pass_gate.py -q
• python3 -m json.tool docs/proof_index.json
• npm run typecheck
• negative empty fixture directory exits 1 with reason=no_fixtures
• negative missing fixture directory exits 1 with reason=fixtures_dir_not_found

Boundaries

• Draft PR only.
• Not ready for review.
• Not merged.
• Provider output is not truth.
• This gate does not prove agent claims are true; it blocks success language when passing evidence pointers or explicit cannot_claim boundaries are missing.