fix(search): split FTS query terms on path separators

[solomonneas]

What

fts_query quoted each whitespace-separated term whole, so a path-bearing query like change src/store/sync.rs became one FTS token that matched nothing. Terms now split on non-alphanumeric separators (keeping _ and $) into quoted tokens joined so any can match; tokens under 2 bytes drop; the existing LIKE fallback still catches degenerate queries.

Why

The co-change bench (receipts in benchmarks/context-cochange/) seeds personalized context packs with change <path> tasks. Path-bearing seeds matched zero symbols, so packs silently fell back to tiny unpersonalized output (646/230 chars). After the fix, recall at the same 4,000-char budget went 0.239 to 0.692 (Go corpus) and 0.200 to 0.504 (Rust corpus), both ahead of an editor-integrated repo-map baseline at matched budget (0.269 / 0.236). TypeScript remains near-zero for both surfaces (import-driven co-change, recorded as the next-experiment seed).

Verification

cargo fmt --check, cargo clippy --all-targets --all-features -- -D warnings, cargo test --all-features, cargo build --release (Brigade receipts). Two new tests fail against the old code. Post-fix bench rerun receipted in the results file.

[coderabbitai]

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