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.

lib/editor/canvas/pen_canvas.dart

@@ -54,6 +54,11 @@ CanvasTool editorToolToCanvas(EditorToolKind kind) => switch (kind) {
       EditorToolKind.eraser => CanvasTool.eraser,
       EditorToolKind.select => CanvasTool.select,
       EditorToolKind.shape => CanvasTool.shape,
+      // The TEXT tool is PDF-editor-only for now (note/slide typed text is a
+      // later increment); the note palette has no text button, so this mapping
+      // is unreachable in practice — fall back to the pen so the switch stays
+      // exhaustive without inventing a PenCanvas typed-text path.
+      EditorToolKind.text => CanvasTool.pen,
     };
 
 class PenCanvas extends StatefulWidget {