feat(event): score CHRONOS occurrence forecasts with a Brier rule

[seonghobae]

Summary

#45 remains ready and merge-first, but is review-blocked (CHANGES_REQUESTED from OpenCode on stale SHA 190a08f; exact-head 293b1f5 has live PostgreSQL, line/branch coverage, and strix SUCCESS). #46–#51 were converted back to draft so they cannot leapfrog #45.

This increment adds CHRONOS occurrence-prediction calibration in existing event_core from protected main 40adac9. It does not recreate in-flight crates or allocate migration 0008.

• Predicted occurrences stay hypothetical and cannot be cast to event instances
• Perfect (1,0,1) forecasts recover Brier 0 against later-observed truth
• Calibrated probabilities beat an always-occur predictor on mixed later truth
• Empty or mismatched streams fail closed

Claim boundary

ADR 0016 forecast-calibration slice only. Not TDT detection, schema-slot extraction, temporal-consistency reasoning, ESEM/DSEM fit, GPU kernels, or encrypted mapping persistence.

Test plan

• RED: prediction_calibration_contract failed with E0432/E0599
• cargo test -p event_core --offline
• cargo clippy -p event_core --all-targets --offline -- -D warnings
• python3 scripts/check_docstrings.py and python3 scripts/validate_documentation.py
• cargo llvm-cov -p event_core lines 343/343; nightly-2026-08-01 --branch 20/20
• Exact-head required Checks; independent review

Do not self-approve or merge. Prefer merge #45 when eligible. Keep this PR draft until then.

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