feat(pen): partial/segment erase + fix side-button while drawing

W4/P0 engine: add engine/stroke_eraser.dart (pure, aspect-corrected) with
whole-stroke `strokeHit` + partial `splitStrokeByCircle`. Grazing a long
stroke now CUTS it into surviving pieces instead of deleting it whole.
Wired through PenCanvas.onEraseStroke (now (index, replacements)) →
pen_editor_screen._eraseStroke (replaceRange); undo/persistence unchanged
(whole-page snapshot). 8 new unit tests; 66/66 pass.

Fix side-button (侧键): _isEraserSignal used `buttons == kSecondaryButton`,
but tip-down + barrel = kStylusContact|kPrimaryStylusButton = 0x03, so the
side button only registered on hover, never while drawing. Now a bitmask
test. (Eraser-end/tilt remain blocked on the silent native badnote/pen
channel — needs on-device native logging.)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/editor/canvas/pen_editor_screen.dart

@@ -247,7 +247,9 @@ class _PenEditorScreenState extends State<PenEditorScreen> {
     _schedulePageSave(_pageIndex, snapshot);
   }
 
-  void _eraseStroke(int index) {
+  /// Replace committed stroke [index] with its surviving pieces after a partial
+  /// (segment) erase. An empty [replacements] list removes the stroke entirely.
+  void _eraseStroke(int index, List<PenStroke> replacements) {
     final list = _strokesByPage[_pageIndex];
     final willMutate = list != null && index >= 0 && index < list.length;
     if (willMutate) {
@@ -256,7 +258,8 @@ class _PenEditorScreenState extends State<PenEditorScreen> {
     }
     setState(() {
       if (list != null && index >= 0 && index < list.length) {
-        final next = List<PenStroke>.of(list)..removeAt(index);
+        final next = List<PenStroke>.of(list)
+          ..replaceRange(index, index + 1, replacements);
         _strokesByPage[_pageIndex] = next;
       }
     });