feat(event): score first-story detections with FAR and miss rates

[seonghobae]

Summary

#37 merged on protected main (babd591). #45 kept ready (0007) but remains review-blocked (no independent APPROVE; still CONFLICTING after #37). Next no-0008 gap: first-story false-alarm/miss calibration in existing event_core (does not recreate TDT/CHRONOS crates, does not wait on #48/#49/#53, and does not allocate 0008).

• refuse_first_story_as_instance never promotes a detection to an instance
• decide_first_story uses an inclusive probability threshold
• first_story_false_alarm_rate and first_story_miss_rate fail closed on empty, mismatched, or single-class streams
• Computed RMSE of known first-story targets is lower under calibrated scores than under an always-first detector

Test plan

• RED: first_story_contract failed with E0432/E0599
• cargo test -p event_core --offline
• cargo clippy -p event_core --all-targets --offline -- -D warnings
• python3 scripts/validate_documentation.py and check_docstrings.py
• exact-head required Checks (do not treat queued evidence as passed)

Do not self-approve or merge. Prefer merge #45 when exact-head required checks and a qualifying independent APPROVE exist. Do not allocate 0008.

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