feat(TRUEREF-0006): implement SQLite FTS5 full-text search engine

- BM25 ranking via SQLite FTS5 bm25() function
- Query preprocessor with wildcard expansion and special char escaping
- Library search with composite scoring (name match, trust score, snippet count)
- Trust score computation from stars, coverage, and source type
- Response formatters for library and snippet results

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/lib/server/search/trust-score.ts

@@ -0,0 +1,41 @@
+/**
+ * Trust score computation for repositories.
+ *
+ * Produces a composite score in [0, 10] that reflects the credibility and
+ * completeness of a repository's documentation.
+ */
+
+import type { Repository } from '$lib/types';
+
+/**
+ * Compute a trust score (0–10) for a repository.
+ *
+ * Score components:
+ * - Stars        : up to 4 points on a log10 scale (10 k stars = 4 pts)
+ * - Doc coverage : up to 3 points (500 snippets = 3 pts)
+ * - Source type  : 1 point for GitHub repos
+ * - Indexed state: 1 point when state is "indexed"
+ * - Description  : 1 point when a description is present
+ */
+export function computeTrustScore(repo: Repository): number {
+	let score = 0;
+
+	// Stars (up to 4 points): log scale, 10k stars ≈ 4 pts.
+	if (repo.stars) {
+		score += Math.min(4, Math.log10(repo.stars + 1));
+	}
+
+	// Documentation coverage (up to 3 points).
+	score += Math.min(3, (repo.totalSnippets ?? 0) / 500);
+
+	// Source type (1 point for GitHub).
+	if (repo.source === 'github') score += 1;
+
+	// Successful indexing (1 point).
+	if (repo.state === 'indexed') score += 1;
+
+	// Has description (1 point).
+	if (repo.description) score += 1;
+
+	return Math.min(10, parseFloat(score.toFixed(1)));
+}