search: keywordSearch score goes negative after 10 results

In `src/core/search.ts`, the `keywordSearch()` fallback calculates scores as `1 - index * 0.1`, which becomes negative after 10 results. Scores should be clamped to `[0, 1]` range.

Additionally, `keywordSearch()` and `fts5Search()` do not implement the `options.minRating` filter — only vector search handles it. This makes filtering behavior inconsistent across search strategies.