feat: select tests for non-Rust inputs via metadata.affected rules

[max-sixty]

cargo-affected selects tests by LLVM line-coverage overlap with the diff, so a test that reads a non-Rust file at runtime (an insta .snap, a doc .md checked by a sync test, an include_str! target) maps to no coverage row. Editing that file overlapped nothing and selected no test: the change shipped but its guarding test never ran.

This adds declarative input→test rules, carried in the manifest cargo-affected already loads via cargo metadata:

[[workspace.metadata.affected.rule]]
globs = ["**/*.snap"]
filterset = "binary_id(=mycrate::integration)"

When a changed path matches a rule's globs, the rule's nextest filterset is resolved with cargo nextest list -E and those tests are force-selected. [[package.metadata.affected.rule]] covers single-crate projects. The filterset speaks the full nextest filter language, so a rule can target a binary, a test-name pattern, or any combination.

What to look at

• src/config.rs (new) — reads and compiles rules from the cargo metadata JSON, resolves matched globs to concrete tests via the filterset. No rules → empty map and no nextest list call, so a project without the table pays nothing.
• src/fingerprint.rs — strips [*.metadata] from the manifest hash so editing a rule doesn't invalidate the coverage cache. Metadata-free manifests still hash by raw bytes (no churn on upgrade); only metadata-carrying ones round-trip through toml_edit, and only the metadata subtree is removed.
• src/selection.rs — rule hits fold into a disjoint config selection category (affected + config + new + stranded = selected), so they never inflate the coverage-overlap counts.

Trade-off

Rules ride in the fingerprinted manifest, but the metadata exclusion makes rule edits cache-neutral. First-time addition of any metadata can move the fingerprint once; the stable guarantee is edit-invariance, covered by manifest_metadata_excluded_from_fingerprint.

Testing

Unit tests cover config parsing (including the [package.metadata] fallback and malformed-table errors), glob compilation, and the fingerprint edit-invariance invariant. Three functional tests exercise the end-to-end miss→fix on a synthetic crate that reads a file at runtime, the committed-added-input path, and the inert-when-no-glob-matches case. Verified live on a real workspace: a .snap edit that previously selected 0 tests now selects the integration suite, and editing a rule keeps cache=hit-exact.

This was written by Claude Code on behalf of max

[cargo-affected-bot]

One small correctness nit on user-facing strings, plus an offer.

The TABLE constant in src/config.rs hardcodes [workspace.metadata.affected], but it's interpolated into five different user-facing messages (parse error, invalid-glob error, builder error, filterset-resolution error, no-tests warning) that fire equally for rules loaded from [package.metadata.affected] via the single-crate fallback. A user on a single-crate project who hits any of these and greps their Cargo.toml for [workspace.metadata.affected] will come up empty. Suggested a one-line fix that mirrors the [*.metadata] shorthand already used in the README's "Input rules" section.

Separately, CLAUDE.md's architecture list doesn't yet mention src/config.rs. Happy to push a one-line addition to that entry if you'd like — there are several open doc-only PRs against CLAUDE.md already (#29, #36, #47), so I held off rather than racing them.