feat(editor): pen canvas + Material You UI

Clean-room reimplementation of Saber's input model: we own the gesture
pipeline so pen draws with real pressure (perfect_freehand), two-finger
pinch zooms/pans, and palm is rejected (stylus-priority, 2nd-pointer
cancels stroke). pdfrx renders one page at a time (PdfPageView, no
gestures) under a shared transform; page-based nav. Material You theme
via dynamic_color (system accent + seed fallback) and a floating tonal
tool palette + page pill. Old pdfrx-overlay spike no longer wired.

lib/editor/pdf/spike_launcher.dart

@@ -8,10 +8,13 @@
 import 'package:file_picker/file_picker.dart';
 import 'package:flutter/material.dart';
 
-import 'spike_editor_pane.dart';
+import '../canvas/pen_editor_screen.dart';
 
-/// Opens a file picker for a PDF, then pushes the spike pane on it.
-/// Pass the user's own large PDF to get a realistic 60fps / pen test.
+/// Opens a file picker for a PDF, then pushes the NEW pen-first canvas editor.
+///
+/// The 🧪 entry now opens the clean-room canvas (lib/editor/canvas/), which
+/// OWNS the gesture pipeline (pressure, pinch-zoom, palm rejection). The old
+/// spike_* files are left in place but no longer wired to this entry.
 Future<void> openM1Spike(BuildContext context) async {
   final result = await FilePicker.platform.pickFiles(
     type: FileType.custom,
@@ -22,13 +25,7 @@ Future<void> openM1Spike(BuildContext context) async {
   if (!context.mounted) return;
   Navigator.of(context).push(
     MaterialPageRoute(
-      builder: (_) => Scaffold(
-        appBar: AppBar(title: const Text('M1 Spike — pen / scroll / zoom')),
-        // denseStrokesAsset is null here: the bundled synthetic-stroke load is
-        // only for the perf bench. For manual on-device testing, draw real ink
-        // and scroll a real large PDF while watching the frame-time HUD.
-        body: SpikeEditorPane(pdfPath: path),
-      ),
+      builder: (_) => PenEditorScreen(pdfPath: path),
     ),
   );
 }