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/engine/stroke_eraser.dart

@@ -0,0 +1,106 @@
+// lib/editor/engine/stroke_eraser.dart
+//
+// Pure stroke-eraser geometry for the BadNote editor (P0 engine layer).
+//
+// Operates on the live `PenStroke`/`PenPoint` model in NORMALIZED page
+// coordinates ([0,1] x [0,1] relative to the page rectangle). It provides two
+// erase modes:
+//
+//   * [strokeHit]            — whole-stroke proximity test (the legacy behavior
+//                              that `pen_canvas._eraseAt` used: any point within
+//                              the eraser circle ⇒ the entire stroke is removed).
+//   * [splitStrokeByCircle]  — partial / segment erase: points inside the eraser
+//                              circle are removed, and each maximal run of
+//                              surviving consecutive points becomes its own
+//                              sub-stroke. A long stroke grazed in the middle is
+//                              cut into two pieces instead of vanishing whole.
+//
+// ASPECT: x and y are each normalized against a different page dimension, so an
+// on-screen circular eraser maps to an ELLIPSE in normalized space. Callers pass
+// [aspect] = pageHeight / pageWidth so the y delta is corrected and the eraser
+// feels round on screen. [aspect] = 1.0 reproduces the legacy (uncorrected,
+// width-normalized) distance.
+
+import '../canvas/pen_stroke.dart';
+
+/// Squared, aspect-corrected normalized distance from ([cx],[cy]) to [p].
+double _dist2(PenPoint p, double cx, double cy, double aspect) {
+  final dx = p.x - cx;
+  final dy = (p.y - cy) * aspect;
+  return dx * dx + dy * dy;
+}
+
+/// True when any point of [stroke] lies within [radius] (normalized, in page-
+/// width fractions) of the eraser center ([cx],[cy]). This is the whole-stroke
+/// hit test — equivalent to the legacy `_eraseAt` proximity check.
+bool strokeHit(
+  PenStroke stroke,
+  double cx,
+  double cy,
+  double radius, {
+  double aspect = 1.0,
+}) {
+  final r2 = radius * radius;
+  for (final p in stroke.points) {
+    if (_dist2(p, cx, cy, aspect) < r2) return true;
+  }
+  return false;
+}
+
+/// Partial erase: remove every point of [stroke] within [radius] of the eraser
+/// center ([cx],[cy]) and return the surviving sub-strokes (preserving color /
+/// width / kind). Each maximal run of >= 2 consecutive surviving points becomes
+/// one sub-stroke; orphaned single survivors are dropped (a 1-point dot left
+/// between two erased gaps is visually negligible and avoids speckle).
+///
+/// Returns:
+///   * `[stroke]`  when nothing is erased (no point hit) — same identity, so the
+///     caller can cheaply detect "no change".
+///   * `[]`        when the whole stroke is erased.
+///   * 1+ new strokes otherwise (the cut pieces).
+List<PenStroke> splitStrokeByCircle(
+  PenStroke stroke,
+  double cx,
+  double cy,
+  double radius, {
+  double aspect = 1.0,
+}) {
+  final r2 = radius * radius;
+  final pts = stroke.points;
+
+  // Fast path: if no point is hit, the stroke is unchanged (return same object).
+  var anyHit = false;
+  for (final p in pts) {
+    if (_dist2(p, cx, cy, aspect) < r2) {
+      anyHit = true;
+      break;
+    }
+  }
+  if (!anyHit) return [stroke];
+
+  final result = <PenStroke>[];
+  var run = <PenPoint>[];
+
+  void flush() {
+    if (run.length >= 2) {
+      result.add(PenStroke(
+        points: List.of(run),
+        color: stroke.color,
+        width: stroke.width,
+        kind: stroke.kind,
+      ));
+    }
+    run = <PenPoint>[];
+  }
+
+  for (final p in pts) {
+    if (_dist2(p, cx, cy, aspect) < r2) {
+      flush(); // hit a gap → close the current surviving run
+    } else {
+      run.add(p);
+    }
+  }
+  flush();
+
+  return result;
+}