feat(f8): search snippet extraction pure core

For library-wide full-text search (the user's #1 named differentiator):
snippetFor() finds the first case-insensitive match of a query in a source
string (PDF text page / typed box / OCR'd handwriting) and returns a windowed
excerpt centered on it, preserving the match offset + length and
truncatedStart/End flags so the results list can render "…ctx **match** ctx…"
and jump to the hit. The full match is always included; near-edge matches don't
over-truncate. Returns null for empty/absent query.

The FTS index + ranking live in the DB (search_indexer, later); this is the
pure, storage-free excerpt math, fully unit-tested.

flutter analyze lib/editor clean; 164/164 tests (+8).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/editor/search/search_snippet.dart

@@ -0,0 +1,73 @@
+// lib/editor/search/search_snippet.dart
+//
+// Pure snippet extraction for library-wide full-text search (F8 — the user's #1
+// named differentiator). Given a source string (a PDF text page, a typed text
+// box, or OCR'd handwriting) and a query, produce a windowed excerpt centered on
+// the first match with the match offset preserved, so the results list can show
+// "…context **match** context…" and jump to the hit.
+//
+// The FTS index / ranking lives in the DB (search_indexer); THIS is the pure,
+// storage-free excerpt math, fully unit-tested.
+
+/// A windowed excerpt around a query match.
+class Snippet {
+  const Snippet({
+    required this.text,
+    required this.matchStart,
+    required this.matchLength,
+    required this.truncatedStart,
+    required this.truncatedEnd,
+  });
+
+  /// The excerpt (a substring of the source).
+  final String text;
+
+  /// Offset of the match WITHIN [text].
+  final int matchStart;
+
+  /// Length of the matched run.
+  final int matchLength;
+
+  /// True when [text] begins before the source start was reached (show a
+  /// leading ellipsis).
+  final bool truncatedStart;
+
+  /// True when [text] ends before the source end (show a trailing ellipsis).
+  final bool truncatedEnd;
+
+  /// Convenience: the matched substring.
+  String get match => text.substring(matchStart, matchStart + matchLength);
+
+  @override
+  String toString() =>
+      '${truncatedStart ? '…' : ''}$text${truncatedEnd ? '…' : ''}'
+      ' [match @$matchStart+$matchLength]';
+}
+
+/// First case-insensitive match of [query] in [source], as a snippet of up to
+/// roughly [window] characters centered on the match. Returns null when [query]
+/// is empty or absent. The full match is always included even if longer than
+/// [window].
+Snippet? snippetFor(String source, String query, {int window = 80}) {
+  if (query.isEmpty || source.isEmpty) return null;
+  assert(window >= 0);
+
+  final matchIndex = source.toLowerCase().indexOf(query.toLowerCase());
+  if (matchIndex < 0) return null;
+
+  final matchLen = query.length;
+  final contextEach = ((window - matchLen) ~/ 2).clamp(0, window);
+
+  var start = matchIndex - contextEach;
+  if (start < 0) start = 0;
+  var end = matchIndex + matchLen + contextEach;
+  if (end > source.length) end = source.length;
+
+  return Snippet(
+    text: source.substring(start, end),
+    matchStart: matchIndex - start,
+    matchLength: matchLen,
+    truncatedStart: start > 0,
+    truncatedEnd: end < source.length,
+  );
+}