fix(bench): anchor pathsMatch to path segments

[costajohnt]

Fixes #943

pathsMatch in src/bench/score.ts accepted a match when either normalized path was a plain string suffix of the other. Two false positives follow from that: a bare Standards.md in expected_files credits Memory/Procedural/Vault Note Standards.md, and an expected docs/api/README.md credits a result of just README.md. Every bench metric (hitsWithin, matched_files, mrr) goes through this function, so both inflate scores.

The README documents expected_files as collection-relative paths as shown by qmd ls, so equality is the contract. This keeps exact match after normalization and one lenient form: the expected path as a whole-segment suffix of the result (nr.endsWith("/" + ne)), which still lets a fixture name docs/readme.md against a result under a longer prefix. The reverse direction (expected longer than result) and mid-filename suffixes no longer match.

This is a deliberate behaviour change: the "suffix match (expected is longer)" case in test/bench-score.test.ts asserted the old behaviour and now asserts false; the result-is-longer case is kept in its anchored form; two cases cover the issue's examples. One scoreResults fixture relied on the loose match (qmd://concepts/a.md vs concepts/a.md) and now uses a real collection prefix.

Tested with bun test test/bench-score.test.ts (19 pass) and bun run lint. CHANGELOG entry under Unreleased.