feat(coverage): rivet coverage --matrix — V&V matrix from repo-status artifacts

[avrabe]

Sub-issue 2 of #188 — the consumer surface for the repo-status schema that landed via #232. Reads repo-status artifacts from the local project and renders a per-repo × per-technique V&V coverage matrix in text, json, markdown, or html.

The cross-repo aggregator (sub-issue 3 of the prior triage decomposition) is still out of scope here because this triage agent's GitHub access is restricted to pulseengine/rivet only.

Acceptance — sub-issue 2 of #188

Per the triage comment on #188 (2026-04-26):

#2. rivet coverage --matrix (single-repo, local) — emits markdown/HTML/JSON for the local rivet.yaml. Acceptance = golden-file tests for each format; CLI flag parsing tested.

Mapped to bullets:

• rivet coverage --matrix flag accepted. New --matrix flag on the existing Coverage clap variant; mutually exclusive with --tests.
• Emits markdown. Pipe table with header + separator. Test: coverage_matrix_markdown.
• Emits HTML. <section><table> fragment with cell-{absent,applied,gated} classes; &/</> escaped. Test: coverage_matrix_html.
• Emits JSON. {command: "coverage-matrix", columns, repos[]} with per-cell {technique, status} records. Test: coverage_matrix_json.
• Single-repo, local rivet.yaml. Reads repo-status artifacts via ProjectContext::load; cross-repo aggregation deferred to sub-issue 3.
• Golden-style tests for each format. 7 integration tests in rivet-cli/tests/cli_commands.rs covering markdown / html / json / text-default rendering, invalid-format diagnostic, --matrix × --tests clap conflict, and empty-project graceful render across all four formats.
• CLI flag parsing tested. coverage_matrix_invalid_format_fails (lists the four valid options) and coverage_matrix_conflicts_with_tests_flag (clap-level mutual exclusion).

Cell semantics

State	Glyph	JSON	Meaning
absent	·	"absent"	Technique not in techniques-applied.
applied	○	"applied"	Applied but not gated in CI.
gated	●	"gated"	In techniques-gated-in-ci.

Columns are the sorted union of techniques-applied ∪ techniques-gated-in-ci across all rows, so the matrix only shows techniques at least one repo cares about.

Docs

• New coverage-matrix topic in rivet docs documenting the surface, cell semantics, authoring repo-status, and the four output formats.
• New schema/vv-coverage topic exposing the schema YAML.

Test plan

• cargo test -p rivet-cli — full suite green
• cargo test -p rivet-core --lib — 896 pass
• cargo clippy -p rivet-cli --all-targets -- -D warnings — clean
• cargo fmt --all -- --check — clean
• rivet validate diagnostics unchanged from origin/main (the 6 pre-existing errors are in the spar-external cross-repo fixture, untouched by this PR)
• Smoke: rivet coverage --matrix --format markdown against a tmpdir with three repo-status artifacts produces the expected table

Closes part of #188. Does not close the issue — sub-issues 3 (cross-repo aggregator), 4 (publishing pipeline), and 5 (per-standard mapping) remain.

Refs: #188
Refs: #184

🤖 Generated with Claude Code — issue-triage agent run 2026-04-29.

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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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