feat(tools): rnote-style toolbar core writing batch

Replace the ad-hoc tool palette with a shared tool system
(EditorToolKind) across the PDF, note and slide editors, and add
the core writing tools.

- Multiple brushes, each remembering its OWN color (rnote-style):
  selecting a brush restores its color, changing color updates only
  that brush, and each brush button shows its current color.
- Select tool: tap-select a committed stroke, drag to move it,
  delete it — persisted and undoable.
- Shape tool: line / rectangle / ellipse / arrow, drawn with a live
  preview and committed as generated PenStrokes (shape_geometry.dart)
  so they reuse stroke rendering, erase, persistence and undo.
- Highlighter + eraser fold into the same tool system.

Text/bookmark/search+OCR/backgrounds/Windows-Ink are later batches
(TODO). Brush opacity still deferred. analyze clean, 302 tests.

lib/editor/canvas/editor_tool.dart

@@ -0,0 +1,53 @@
+// lib/editor/canvas/editor_tool.dart
+//
+// The shared tool model for the pen-first editors (PDF, note, slide). Replaces
+// the scattered per-editor booleans (`_selectTextMode`, `_placeLinkMode`, the
+// old `CanvasTool` pen/highlighter/eraser triad) with ONE active-tool enum so
+// every editor reasons about "which tool is active" the same way.
+//
+// [EditorToolKind] is the core, render-path-independent set shared by all three
+// editors. The PDF editor layers TWO extra page-anchored tools on top
+// (select-text and place-scratch-link) that the PenCanvas editors don't have —
+// 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.
+
+/// The shared inking/editing tools available on every pen-first canvas.
+enum EditorToolKind {
+  /// Freehand drawing with the currently-selected [BrushKind] (fountain pen,
+  /// ballpoint, or pencil). Each brush carries its own remembered color.
+  brush,
+
+  /// Freehand drawing with the highlighter brush (its own color + flat width).
+  highlighter,
+
+  /// Stroke eraser (partial / whole-stroke per PenConfig).
+  eraser,
+
+  /// Cursor / selection tool: tap a committed stroke to select it, drag the
+  /// selection to translate it, delete to remove it.
+  select,
+
+  /// Shape tool: pen-drag previews a [ShapeKind] from start→current and commits
+  /// it as a generated [PenStroke] on release.
+  shape,
+}
+
+/// The shapes the [EditorToolKind.shape] tool can draw. Each is generated as a
+/// plain [PenStroke] (a polyline) so it reuses stroke rendering, persistence,
+/// erase, and undo with no new model — see `shape_geometry.dart`.
+enum ShapeKind {
+  /// Straight line: 2 points (start → end).
+  line,
+
+  /// Axis-aligned rectangle: 5-point closed polyline (start corner → end corner).
+  rectangle,
+
+  /// Ellipse inscribed in the start→end bounding box: ~48 sampled points.
+  ellipse,
+
+  /// Arrow: shaft (start → end) plus two arrowhead segments at the end.
+  arrow,
+}