feat(search): add match-centred snippet extraction

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

src/search/snippet.ts

@@ -0,0 +1,45 @@
+/**
+ * Match-centred excerpt extraction for search results.
+ */
+
+const DEFAULT_MAX_LENGTH = 160;
+const CONTEXT_BEFORE = 40;
+
+/**
+ * Extracts an excerpt of `text` centred on the earliest match offset.
+ *
+ * `matchedOffsets` are offsets into the ORIGINAL text, as produced by
+ * `scoreField`. When no offsets are supplied the excerpt falls back to a
+ * head truncation.
+ */
+export function extractSnippet(
+  text: string,
+  matchedOffsets: number[],
+  maxLength: number = DEFAULT_MAX_LENGTH
+): string {
+  if (!text) {
+    return "";
+  }
+
+  if (text.length <= maxLength) {
+    return text.trim();
+  }
+
+  let start = 0;
+  if (matchedOffsets.length > 0) {
+    const earliest = Math.min(...matchedOffsets);
+    start = Math.max(0, Math.min(earliest - CONTEXT_BEFORE, text.length - maxLength));
+  }
+
+  const end = Math.min(text.length, start + maxLength);
+  let snippet = text.slice(start, end).trim();
+
+  if (start > 0) {
+    snippet = `...${snippet}`;
+  }
+  if (end < text.length) {
+    snippet = `${snippet}...`;
+  }
+
+  return snippet;
+}