feat(storage): one-time SQLite to sidecar migration

Phase 5. On first launch with a valid vault, migrate legacy SQLite
data into vault sidecars so nothing is lost on upgrade.

- SqliteToSidecarMigrator: documents (+ per-page ink, scratch-links
  + scratchpads, bookmarks) -> notebook folder + <file>.badnote.json;
  notes (+ strokes) -> notebook.badnote.json. Reuses existing JSON.
- Idempotent (skips already-migrated targets); missing source files
  still get their annotations migrated.
- DB relocates to <vault>/.badnote/index.sqlite; the legacy DB is
  renamed to .premigration ONLY after a successful pass, so a failed
  migration leaves data intact and the run-once flag unset.
- main.dart runs it once, gated on vaultMigrationDone.

Golden migrator tests (seeded legacy DB -> sidecars, idempotent
re-run, legacy preserved). analyze clean, tests green.

lib/storage/badnote_sidecar.dart

@@ -221,10 +221,13 @@ class BadnoteSidecar {
     Map<int, List<SidecarHighlight>>? highlights,
     List<Bookmark>? bookmarks,
     List<SidecarScratchLink>? scratchLinks,
+    Map<int, String>? legacyAnnotations,
+    this.legacyId,
   })  : strokes = strokes ?? <int, List<EditorStroke>>{},
         highlights = highlights ?? <int, List<SidecarHighlight>>{},
         bookmarks = bookmarks ?? <Bookmark>[],
-        scratchLinks = scratchLinks ?? <SidecarScratchLink>[];
+        scratchLinks = scratchLinks ?? <SidecarScratchLink>[],
+        legacyAnnotations = legacyAnnotations ?? <int, String>{};
 
   /// Schema version (`badnoteSidecarVersion`).
   final int version;
@@ -253,6 +256,19 @@ class BadnoteSidecar {
   final List<Bookmark> bookmarks;
   final List<SidecarScratchLink> scratchLinks;
 
+  /// Raw legacy per-page `annotation_json` blobs preserved verbatim from the
+  /// DEAD pre-editor `annotations` SQLite table (keyed by page number). Populated
+  /// only by the one-time SQLite→sidecar migration so no legacy data is silently
+  /// dropped; the live editor ignores it. Empty for all freshly authored
+  /// sidecars.
+  final Map<int, String> legacyAnnotations;
+
+  /// The legacy SQLite row id this sidecar was migrated from (a `documents.id`
+  /// or `notes.id`). Set ONLY by the one-time migration; it makes the migration
+  /// idempotent (a re-run recognizes an already-migrated item by this id even if
+  /// its folder name collided). Null for all freshly authored sidecars.
+  final String? legacyId;
+
   Map<String, dynamic> toJson() => {
         'badnoteSidecarVersion': version,
         if (sourceFile != null) 'sourceFile': sourceFile,
@@ -274,6 +290,12 @@ class BadnoteSidecar {
         },
         'bookmarks': bookmarks.map((b) => b.toJson()).toList(),
         'scratchLinks': scratchLinks.map((s) => s.toJson()).toList(),
+        if (legacyAnnotations.isNotEmpty)
+          'legacyAnnotations': {
+            for (final entry in legacyAnnotations.entries)
+              entry.key.toString(): entry.value,
+          },
+        if (legacyId != null) 'legacyId': legacyId,
       };
 
   factory BadnoteSidecar.fromJson(Map<String, dynamic> json) {
@@ -316,6 +338,18 @@ class BadnoteSidecar {
       scratchLinks: ((json['scratchLinks'] as List<dynamic>?) ?? const [])
           .map((e) => SidecarScratchLink.fromJson(e as Map<String, dynamic>))
           .toList(),
+      legacyAnnotations: () {
+        final raw = json['legacyAnnotations'];
+        final out = <int, String>{};
+        if (raw is Map) {
+          raw.forEach((key, value) {
+            final page = int.tryParse(key.toString());
+            if (page != null && value is String) out[page] = value;
+          });
+        }
+        return out;
+      }(),
+      legacyId: json['legacyId'] as String?,
     );
   }
 }