feat(rivet-core): CoverageStore for external coverage evidence (witness consumer)

[avrabe]

Summary

Adds a coverage_evidence module to rivet-core that consumes branch / MC/DC / line coverage evidence emitted by external tools (initially the upstream emitter at pulseengine/witness, schema URL https://pulseengine.eu/witness-rivet-evidence/v1).

This is a separate concept from the existing coverage.rs traceability-rule coverage:

• coverage.rs (existing) — models artifact-graph rule satisfaction
• coverage_evidence.rs (new) — ingests external test-run evidence for downstream rendering

The on-disk YAML schema is documented in the upstream emitter at pulseengine/witness/docs/research/rivet-evidence-consumer.md.

What's added

• CoverageEvidence (per-artifact, per-run): artifact, coverage_type (branch | mcdc | line), total, covered, percentage, hits, uncovered branch ids
• CoverageStore: insert, latest_for, history_for, summary; mirrors ResultStore semantics (newest-first ordering, summary from latest run)
• load_evidence(file) and load_evidence_dir(dir): rejects unknown schema URLs, skips invalid files in dir-mode with a stderr warning
• Error::CoverageEvidence variant for typed error propagation

End-to-end flow (post-merge + rivet release)

1. This PR merges → CoverageStore ships in a future rivet release
2. witness emitter writes evidence YAML matching the v1 schema
3. rivet ingests via load_evidence_dir → CoverageStore
4. Downstream rendering (dashboard, reports) reads via latest_for / summary

Test plan

• cargo test -p rivet-core --lib — 780 passed (9 new)
• cargo clippy -p rivet-core -- -D warnings — clean
• cargo deny check — pass
• cargo fmt — clean
• CI green (this PR triggers it)

Reviewer notes

• Schema URL is fixed at v1 with version-bump rejection in load_evidence. Future versions will need a discriminator and explicit migration.
• load_evidence_dir is intentionally permissive: invalid files emit a stderr warning and are skipped, so a directory mixing rivet evidence with other YAML doesn't fail the whole load. Strict mode would be a follow-up if needed.

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 95.47325% with 11 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
rivet-core/src/coverage_evidence.rs	95.47%	11 Missing ⚠️

📢 Thoughts on this report? Let us know!

[github-actions]

📐 Rivet artifact delta

No artifact changes in this PR. Code-only changes (renderer, CLI wiring, tests) don't touch the artifact graph.

[github-actions]

⚠️ Performance Alert ⚠️

Possible performance regression was detected for benchmark 'Rivet Criterion Benchmarks'.
Benchmark result of this commit is worse than the previous benchmark result exceeding threshold 1.20.

Benchmark suite	Current: a8cad5c	Previous: c7d5664	Ratio
store_lookup/10000	471434 ns/iter (± 1467)	378215 ns/iter (± 1969)	1.25
traceability_matrix/1000	60828 ns/iter (± 179)	43287 ns/iter (± 135)	1.41
query/10000	116345 ns/iter (± 918)	91697 ns/iter (± 323)	1.27

This comment was automatically generated by workflow using github-action-benchmark.