test(discovery): check planted canaries and gate the run

[pengyuzhang]

Stacked on #63.

Two things that decide whether a run may pass, kept apart from the measurement they sit on.

The canary check is deliberately crude

It does not know how redaction is implemented, does not walk the asset model, and does not care which field a value came from. It searches the serialized snapshot — every byte the collector would have written to a file or posted to a server — for each planted string.

That crudeness is the point. A credential that leaked into a field the harness does not model is exactly the one a smarter, model-aware check would miss.

Any hit fails the run outright, regardless of every other score. A collector that finds every tool and leaks one token has not had a good run.

Two details worth reviewing:

• A canary with no recorded value is reported as unplanted, not clean. A check that never ran is not a check that passed.
• The context line around a hit masks the value it reports. Printing it into a document that then gets shared would repeat exactly the mistake being reported.

The gate

Fails on: a leaked canary, a dirty baseline, an unexplained error, any DUP, a missed review-queue entry, or recall below the last accepted run for that OS.

Recall is compared against history, not an absolute threshold. A real endpoint is not perfectly reproducible — a background updater can change a version mid-run — so an absolute number would either be set so low it gates nothing or would flap. Everything else is binary, because a leaked credential or an invented asset is not weather.

Verification

$ python3 -m unittest discover -s tests -t . -q
Ran 58 tests in 0.039s
OK

Ten new tests, each injecting exactly one condition into the recorded run: a leaked canary, an unplanted one, a dirty baseline, an unexplained error, a repeated asset id, and a recall regression against a previous score. On the recorded run itself:

canaries: 6 planted, 0 leaked, clean
gate:     FAILED - duplicates

which is correct: that fixture carries an injected duplicate.