feat(mutants): canonical cargo-mutants template + docs + schema fields (#185)

[avrabe]

Closes #185

Summary

First in-scope cut at the cargo-mutants generalization story. Three deliverables, mapping 1:1 to the in-scope acceptance bullets confirmed in the 2026-04-26 triage comment on #185:

Acceptance bullet	Where it landed
Extract rivet's cargo-mutants config into a reusable template (workflow YAML + mutants.toml)	templates/cargo-mutants/{mutants.toml, mutants.yml, README.md}
docs/mutation-testing.md — when to run, ASIL/DAL score targets, marking unreachable mutants	docs/mutation-testing.md
Schema records mutation_score on TEST-* artifacts	schemas/score.yaml — mutation-score-target on test-spec (suite floor) and mutation-score + mutants-tested / killed / missed / timeout / unviable counts on test-exec (measured run)

What's in the template

• mutants.toml — minimum_test_timeout=60, additional_cargo_test_args=["--lib"], exclude_globs for target/, vendor/, proofs/, verus/, fuzz targets, plus a starter skip_calls list (tracing::trace, tracing::debug).
• mutants.yml — nightly + manual-dispatch GitHub Actions workflow with a per-crate matrix, 90-min per-shard timeout, and mutants.out/ artifact upload.
• README.md — quickstart for adopters, with the three operating modes (pre-commit off, pre-push smoke, CI nightly) called out explicitly.

Score-target ladder (in docs/mutation-testing.md)

Safety level	Floor
QM / DAL E	optional
ASIL A / DAL D	≥ 0.70
ASIL B / DAL C	≥ 0.80
ASIL C / DAL B	≥ 0.85
ASIL D / DAL A	≥ 0.90

Recorded on artifacts via mutation-score-target (target) and mutation-score (measured); the doc spells out the relation to mutants-tested / killed / missed / timeout / unviable.

Out of scope

These bullets from #185 cannot be addressed from a session scoped to pulseengine/rivet:

• Open follow-up adoption issues on kiln, loom, gale, meld — needs broader org access.
• Cross-repo dashboard view — depends on V&V coverage matrix dashboard via rivet coverage (per-commit, per-repo, per-technique) #188.

The issue should remain open until those are filed if Closes #185 is treated as too aggressive; happy to re-tag as Refs: instead.

Verification

• cargo test --release -p rivet-core --lib — 857 passed, 0 failed.
• cargo test --release -p rivet-core --test integration --test schema_agent_pipelines --test yaml_roundtrip — 14 passed, 0 failed.
• cargo fmt --all -- --check — clean.
• rivet validate — diagnostics identical to origin/main (6 pre-existing errors in the spar-external fixture; unchanged here).
• Synthetic project loading schemas: [common, score] accepts both mutation-score-target: 0.9 on test-spec and the full mutation-score + counts block on test-exec.

Test plan

• Schema parses (rivet-core embedded include_str!)
• Lib + integration tests green
• No new validate diagnostics
• CI green
• Maintainer signs off on the score-target ladder
• Cross-repo adoption issues filed separately (see "Out of scope")

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Generated by Claude Code — issue-triage agent run 2026-04-27.

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Generated by Claude Code

[github-actions]

📐 Rivet artifact delta

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

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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