fix(corpus): trust verified OpenRouter free aliases

[Audacity88]

Summary

• Base branch: master
• What changed and why:
  • Preserve a live OpenRouter marker when an exact :free alias reports zero prompt and completion prices.
  • Allow that verified marker to override the commercial-vendor placeholder heuristic.
  • Add regressions proving that suffix-only and non-:free commercial aliases remain blocked.
• Scope boundary: This does not trust arbitrary zero-price feed rows, weaken post-call cost verification, or change fallback behavior.
• Blast radius: Public corpus generation and free-model route eligibility. Runtime paid/free telemetry enforcement is unchanged.
• Linked issue(s): None.

Validation Evidence

Commands run:

$ python3 scripts/tests/build-public-corpus-tests.py
...
Ran 3 tests in 0.002s

OK

$ python3 -m py_compile scripts/build-public-corpus.py scripts/tests/build-public-corpus-tests.py
(no output)

$ python3 scripts/build-public-corpus.py --out-dir <temporary-directory> --date 2026-08-13
wrote corpus (2579 models, 79 routable) + pricing (2579 models) [generated 2026-08-13]

$ zoder exec --model google/gemma-4-31b-it:free --require-free --dry-run --explain
[route] pinned google/gemma-4-31b-it:free (explicit -m); fallbacks suppressed by operator pin
[dry-run] would call google/gemma-4-31b-it:free via openrouter-review (chain: google/gemma-4-31b-it:free)

• Beyond CI: Rebuilt the corpus from the current public feeds and confirmed google/gemma-4-31b-it:free became free=true, paid=false, and route_candidate=true. The Zoder dry route then accepted it under --require-free without making an inference call.
• Intentionally skipped: The full Rust workspace gate. This patch changes only the Python corpus generator and its Python regression tests; CI remains authoritative for the unchanged Rust workspace.

Security & Privacy Impact

• New permissions, capabilities, or filesystem access scope? No
• New external network calls? No
• Secrets / tokens / credentials handling changed? No
• PII / real identities / personal data in diff, tests, fixtures, or docs? No

The pre-call exception is deliberately narrow: the exact model ID must end in :free, and OpenRouter's live feed must independently report both prices as zero. Post-call cost and fallback verification remains unchanged.

Compatibility

• Backward compatible? Yes
• Config / env / CLI surface changed? No

Rollback

Low-risk: revert the commit. The previous commercial-vendor heuristic will again block these aliases until manually classified.