feat(pdf): typed-text tool (Windows-Ink friendly)

Add a text-annotation tool to the PDF editor. With the text tool a
pen-tap, or a mouse double-click, drops a text box at that normalized
page point and focuses a real Flutter TextField — so the OS IME and the
Windows-Ink handwriting panel feed it (device-validated). Tapping an
existing box re-opens it; clearing it deletes it.

- SidecarText {nx, ny, text, fontSize (page-relative), color} per page,
  glued under zoom; stored in the sidecar `texts` field (back-compat
  missing -> none), saved via scheduleTextsSave and loaded on open.
- Rendered in pageOverlaysBuilder at the scaled position.

PDF editor only for now (note text later). analyze clean, 397 tests.

test/badnote_sidecar_test.dart

@@ -76,6 +76,21 @@ BadnoteSidecar _fullSidecar() => BadnoteSidecar(
           SidecarHighlight(l: 0.12, t: 0.20, r: 0.88, b: 0.235, color: 0xFFFFEB3B),
         ],
       },
+      texts: {
+        0: const [
+          SidecarText(
+            id: 't0',
+            nx: 0.25,
+            ny: 0.33,
+            text: 'Hello 手写',
+            fontSize: 0.04,
+            color: 0xFF112233,
+          ),
+        ],
+        3: const [
+          SidecarText(id: 't1', nx: 0.5, ny: 0.6, text: 'second'),
+        ],
+      },
       bookmarks: [
         Bookmark(
           id: 'bm1',
@@ -131,6 +146,11 @@ void main() {
     // Highlights.
     expect(reparsed.highlights[0], original.highlights[0]);
 
+    // Typed-text annotations (SidecarText value equality).
+    expect(reparsed.texts.keys.toSet(), {0, 3});
+    expect(reparsed.texts[0], original.texts[0]);
+    expect(reparsed.texts[3], original.texts[3]);
+
     // Bookmarks (Bookmark value equality via freezed).
     expect(reparsed.bookmarks, original.bookmarks);
 
@@ -214,11 +234,64 @@ void main() {
     final reparsed = _roundTrip(BadnoteSidecar());
     expect(reparsed.strokes, isEmpty);
     expect(reparsed.highlights, isEmpty);
+    expect(reparsed.texts, isEmpty);
     expect(reparsed.bookmarks, isEmpty);
     expect(reparsed.scratchLinks, isEmpty);
     expect(reparsed.version, kBadnoteSidecarVersion);
   });
 
+  test('text annotation round-trips (page, nx/ny, text, fontSize, color)', () {
+    final original = BadnoteSidecar(texts: {
+      2: const [
+        SidecarText(
+          id: 'tx1',
+          nx: 0.18,
+          ny: 0.42,
+          text: 'A typed note 手写测试',
+          fontSize: 0.035,
+          color: 0xFFAB12CD,
+        ),
+      ],
+    });
+    final reparsed = _roundTrip(original);
+    expect(reparsed.texts.keys.toSet(), {2});
+    final t = reparsed.texts[2]!.single;
+    expect(t.id, 'tx1');
+    expect(t.nx, 0.18);
+    expect(t.ny, 0.42);
+    expect(t.text, 'A typed note 手写测试');
+    expect(t.fontSize, 0.035);
+    expect(t.color, 0xFFAB12CD);
+    // Full SidecarText value equality.
+    expect(reparsed.texts[2], original.texts[2]);
+  });
+
+  test('text JSON is byte-compatible with SidecarText.toJson', () {
+    const tx = SidecarText(id: 't0', nx: 0.25, ny: 0.33, text: 'hi');
+    final sidecar = BadnoteSidecar(texts: {1: const [tx]});
+    final json = sidecar.toJson();
+    final pageList = (json['texts'] as Map)['1'] as List;
+    expect(pageList.single, tx.toJson());
+  });
+
+  test('missing texts decodes to empty (back-compat)', () {
+    // A sidecar authored before the texts field existed (no `texts` key).
+    final json =
+        jsonDecode(jsonEncode(BadnoteSidecar().toJson())) as Map<String, dynamic>;
+    expect(json.containsKey('texts'), isFalse,
+        reason: 'empty texts omitted (no key bloat for legacy sidecars)');
+    final reparsed = BadnoteSidecar.fromJson(json);
+    expect(reparsed.texts, isEmpty);
+  });
+
+  test('SidecarText defaults fill in for a minimal JSON blob', () {
+    // Only the required fields present; fontSize/color/text fall to defaults.
+    final t = SidecarText.fromJson(const {'id': 'a', 'nx': 0.1, 'ny': 0.2});
+    expect(t.text, '');
+    expect(t.fontSize, 0.03);
+    expect(t.color, 0xFF000000);
+  });
+
   test('paragraph-anchored bookmark round-trips its anchor + char index', () {
     final original = BadnoteSidecar(bookmarks: [
       Bookmark(