Versioned calibration corpus for salience

[riktar]

Extend the short-turn calibration regression test into a versioned corpus file: phatic, substantive, cue-bearing, and entity-rich turns with expected verdicts. Any salience fork must show its diff against this corpus.

• tests/fixtures/calibration_corpus.yaml (text, role, expected verdict, rationale) — start from the cases in the existing regression test, target ≥ 60 rows
• Parametrized test consuming the corpus in CI

Contributing note: how to add rows, when a change requires a new salience@vN