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/editor_tool.dart

@@ -11,8 +11,9 @@
 // those remain editor-local because they ride pdfrx's text layer / the page
 // overlay, not the ink capture path. See `selectTextOrLink` note below.
 //
-// TODO(toolbar-batch-2): text/typing tool, bookmark-to-paragraph, search+OCR,
-// templates, Windows Ink — later batches add kinds here.
+// TODO(toolbar-batch-2): bookmark-to-paragraph, search+OCR, templates — later
+// batches add kinds here. (The typed-text tool now exists as [EditorToolKind.
+// text], PDF-only for now; see the `text` doc below.)
 
 /// The shared inking/editing tools available on every pen-first canvas.
 enum EditorToolKind {
@@ -33,6 +34,13 @@ enum EditorToolKind {
   /// Shape tool: pen-drag previews a [ShapeKind] from start→current and commits
   /// it as a generated [PenStroke] on release.
   shape,
+
+  /// Typed-text tool (PDF editor only for now): a pen-tap OR a mouse
+  /// double-click on a page drops a text box at that normalized point and
+  /// focuses a real Flutter text field for input (so the OS IME / Windows-Ink
+  /// handwriting panel works). Committed boxes render glued to the page and are
+  /// re-editable; an empty box deletes itself on blur.
+  text,
 }
 
 /// The shapes the [EditorToolKind.shape] tool can draw. Each is generated as a