fix(affected): exclude fixture paths from the test-file heuristic

[solomonneas]

What

affected reported fixture sources as runnable tests: tests/fixtures/golden/mixed/src/bin/graphtrail-mcp.rs came back in affected_tests because is_test_file counts any path with a tests-like segment. Paths under fixtures/, fixture/, golden/, snapshots/, or testdata/ are now non-tests unless the file name itself matches a test-name pattern. testdata also leaves the generic test-dir list (Go fixture convention).

dead_code filtered candidates with the same predicate, which would have let fixture symbols (intentionally uncalled) flood the candidate list - on this repo, 8 of 60 candidates. It now filters through a broader test-support predicate (tests or fixture dirs), keeping fixtures out of both reports for opposite reasons: not runnable tests for affected, not dead code for dead_code.

Why

An affected-test calibration bench over 17 historical commits of this repo (receipts in benchmarks/affected-calibration/) surfaced the misclassification. The same bench returned a null result on traversal depth: recall identical at depth 1/3/5, so the depth default stays.

Verification

cargo fmt --check, cargo clippy --all-targets --all-features -- -D warnings, cargo test --all-features, cargo build --release (Brigade receipts). E2E on the bench corpus: affected src/store/db.rs src/mcp.rs no longer lists the fixture file; dead-code candidates drop 60 to 52 with 0 fixture paths.

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