test(baseline): close 9 mutation-test gaps (#35)

[Metbcy]

Summary

Closes 9 of the 9 surviving mutants on `src/baseline.rs` found in the issue #35 round-1 mutation audit. Pure test additions; no source changes.

Mutants closed

From `cargo mutants --in-place --no-shuffle --file src/baseline.rs --timeout 60` (round-1 baseline: 54 mutants tested in 24m, 9 missed, 42 caught, 3 unviable):

line:col	mutant	catching test
151:37	replace `&&` with `||` in `from_value_inner` (typosquats arm)	`typosquat_key_requires_both_purl_and_closest_nonempty`
176:37	replace `&&` with `||` in `from_value_inner` (maintainer_age arm)	`maintainer_age_key_requires_both_purl_and_contributor_nonempty`
176:20	delete `!` on `purl.is_empty()`	(same)
176:40	delete `!` on `contrib.is_empty()`	(same)
320:9	delete `!` in `apply` maintainer_age retain	`apply_drops_matched_maintainer_age_and_keeps_unmatched`
414:26	replace `||` with `&&` in `add_suppression_full`	`add_suppression_full_writes_object_form_with_expires_only` + `_with_reason_only`
429:12	delete `!` on `parent.as_os_str().is_empty()`	`add_suppression_full_writes_object_form_with_expires_only` (nested path exercises this)
523:5	replace `doc_kind` with `""`	`add_suppression_full_error_names_actual_root_type` + `doc_kind_maps_each_json_variant_to_its_label`
523:5	replace `doc_kind` with `"xyzzy"`	(same)

Verification

• `cargo test --lib baseline::` → 32 passed, 0 failed (was 25)
• `cargo clippy --all-targets -- -D warnings` → clean
• Re-running `cargo mutants --in-place --file src/baseline.rs` on this branch is in flight; will append outcome to the docs audit log in a follow-up PR alongside the other module recheck results.

Notes

• Each test's docstring names the `line:col` of the original mutant, so future mutants runs that revive any of these can be traced back to the test that should have caught it.
• The module audit log in `docs/src/internals/mutation-testing.md` will be updated in a follow-up PR once all round-2 modules are landed (typosquat, license, maintainer next).

Part of #35.

[github-actions]

Coverage report

Line coverage: 84.5% (9845 / 11644 lines)

Full lcov report available as workflow artifact coverage-lcov: download from this run.

_{v0.9.8 introduces this report; --fail-under-lines will be added once coverage is visible across 2–3 releases.}

[Metbcy]

Mutants recheck on src/baseline.rs with this branch's tests applied:

Found 54 mutants to test
ok       Unmutated baseline in 4s build + 32s test
54 mutants tested in 31m: 51 caught, 3 unviable

0 surviving (down from 9 in round 1). All 9 logic survivors closed by the 7 new tests in this PR. Ready when you are.