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/sidecar_repository_test.dart

@@ -161,6 +161,48 @@ void main() {
     after.dispose();
   });
 
+  test('typed-text annotations persist + restore on reopen', () async {
+    final repo = await SidecarRepository.open(src, debounce: _fast);
+    repo.scheduleTextsSave(2, const [
+      SidecarText(
+        id: 'tx1',
+        nx: 0.2,
+        ny: 0.3,
+        text: 'glued note',
+        fontSize: 0.04,
+        color: 0xFF112233,
+      ),
+    ]);
+    await repo.flush();
+    repo.dispose();
+
+    final after = await SidecarRepository.open(src, debounce: _fast);
+    final t = after.loadedTexts[2]!.single;
+    expect(t.id, 'tx1');
+    expect(t.nx, 0.2);
+    expect(t.ny, 0.3);
+    expect(t.text, 'glued note');
+    expect(t.fontSize, 0.04);
+    expect(t.color, 0xFF112233);
+    after.dispose();
+  });
+
+  test('clearing all text on a page removes the page entry (empty-on-blur)',
+      () async {
+    final repo = await SidecarRepository.open(src, debounce: _fast);
+    repo.scheduleTextsSave(
+        0, const [SidecarText(id: 'a', nx: 0.1, ny: 0.1, text: 'x')]);
+    await repo.flush();
+    // The box is emptied + deleted (empty-on-blur), leaving no texts on page 0.
+    repo.scheduleTextsSave(0, const []);
+    await repo.flush();
+    repo.dispose();
+
+    final after = await SidecarRepository.open(src, debounce: _fast);
+    expect(after.loadedTexts.containsKey(0), isFalse);
+    after.dispose();
+  });
+
   test('deleting a scratch link removes it and its scratchpad', () async {
     final repo = await SidecarRepository.open(src, debounce: _fast);
     repo.scheduleScratchLinkUpsert(const ScratchLink(