fix(ck-engine): apply path scope before top_k in semantic search

[runonthespot]

Problem

`semantic_search_v3` computed cosine similarity for every chunk in the index, sorted them, took the global top_k, and only THEN applied the path filter. With a whole-codebase index plus a narrow `path=` query, the global top_k could be entirely consumed by chunks outside the requested scope — leaving the path filter with nothing to keep and returning an empty result set even though the in-scope file had great matches.

Found via code review (task #10 in the session task list).

Fix

1. Build a `PathScope` (All / Dir / File) once up-front with the target canonicalized once.
2. Apply scope at sidecar-collection time — before we even read embeddings or compute similarities. Earliest possible filter.
3. Remove the now-redundant per-result path check from the iteration loop, which had also been re-canonicalizing `options.path` on every iteration.

Tests

• 3 `PathScope` unit tests (`All` / `Dir` / `File`). Run under `--no-default-features` too — no embedder dependency.
• 1 integration regression test `test_scoped_search_does_not_lose_results_to_global_top_k`: indexes 8 noisy files about "database connection" in `noisy/`, plus 1 in-scope file in `scoped/`, then searches scope=`scoped/` with `top_k=3`. Pre-fix: empty result. Post-fix: in-scope file present. Gated on `fastembed` since it needs a real embedder.

Test plan

• `cargo test -p ck-engine --no-default-features` — 18 passed (was 15)
• `cargo clippy -p ck-engine --no-default-features --all-targets -- -D warnings` — clean
• `cargo fmt --all --check` — clean
• CI green on this PR (fastembed-feature test also passes)

🤖 Generated with Claude Code