feat(f8): search ranking + multi-source aggregation

scoreText (more normalized occurrences rank higher; earlier first match breaks
ties) and rankHits (score every source, drop non-matches, attach a display
snippet of the original text, sort best-first with an explicit input-order
tiebreak since Dart's sort isn't stable). The search_indexer's pure ranking
core, making search_text + search_snippet load-bearing.

flutter analyze lib/editor clean; 223/223 tests (+9).

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

test/search_ranking_test.dart

@@ -0,0 +1,63 @@
+// Tests for search ranking + aggregation (F8).
+
+import 'package:flutter_test/flutter_test.dart';
+
+import 'package:badnote/editor/search/search_ranking.dart';
+
+void main() {
+  group('scoreText', () {
+    test('0 for empty query or no match', () {
+      expect(scoreText('hello world', ''), 0);
+      expect(scoreText('hello world', 'zzz'), 0);
+      expect(scoreText('', 'x'), 0);
+    });
+
+    test('more occurrences score higher', () {
+      final one = scoreText('cat dog', 'cat');
+      final three = scoreText('cat cat cat', 'cat');
+      expect(three, greaterThan(one));
+    });
+
+    test('earlier first match breaks ties (same count)', () {
+      final early = scoreText('needle then padding padding', 'needle');
+      final late = scoreText('padding padding then needle', 'needle');
+      expect(early, greaterThan(late));
+    });
+
+    test('a match at position 0 gives earliness 1 (score = count + 1)', () {
+      expect(scoreText('cat', 'cat'), closeTo(2.0, 1e-9)); // 1 occ + 1.0
+    });
+  });
+
+  group('rankHits', () {
+    test('drops non-matches and orders best-first', () {
+      final hits = rankHits({
+        'p1': 'one mention of fox',
+        'p2': 'fox fox fox everywhere', // 3 occ → highest
+        'p3': 'nothing relevant here',
+      }, 'fox');
+      expect(hits.map((h) => h.ref), ['p2', 'p1']);
+      expect(hits.first.score, greaterThan(hits.last.score));
+    });
+
+    test('each hit carries a display snippet of the original text', () {
+      final hits = rankHits({'p1': 'The quick brown fox jumps'}, 'fox');
+      expect(hits, hasLength(1));
+      expect(hits.first.snippet.match, 'fox');
+      expect(hits.first.ref, 'p1');
+    });
+
+    test('equal scores keep input order (explicit tiebreak)', () {
+      final hits = rankHits({
+        'a': 'match',
+        'b': 'match',
+      }, 'match');
+      expect(hits.map((h) => h.ref), ['a', 'b']);
+      expect(hits[0].score, hits[1].score);
+    });
+
+    test('empty query → no hits', () {
+      expect(rankHits({'a': 'anything'}, ''), isEmpty);
+    });
+  });
+}