Add playable core UI: board painter, drag-and-drop, HUD, result overlay

CustomPainter board with gems/ghost/clear-flash, finger-lifted drag
with snap preview, combo text effect, HUD chips, phase overlays with
rescue stubs, demo stage. E2E widget test drives a real drag gesture.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

lib/ui/widgets/board_geometry.dart

@@ -0,0 +1,41 @@
+import 'dart:math' as math;
+import 'dart:ui';
+
+import '../../game/models/grid.dart';
+import '../../game/models/piece.dart';
+
+/// Pure mapping between board-local pixels and grid cells.
+class BoardGeometry {
+  const BoardGeometry({required this.boardSize});
+
+  final double boardSize;
+
+  double get cellSize => boardSize / GridState.size;
+
+  Rect cellRect(int x, int y) =>
+      Rect.fromLTWH(x * cellSize, y * cellSize, cellSize, cellSize);
+
+  /// Cell under a local point, or null outside the board.
+  (int, int)? cellAt(Offset local) {
+    if (local.dx < 0 || local.dy < 0) return null;
+    if (local.dx >= boardSize || local.dy >= boardSize) return null;
+    return (local.dx ~/ cellSize, local.dy ~/ cellSize);
+  }
+
+  /// Nearest anchor for a piece whose visual top-left sits at [pieceTopLeft],
+  /// clamped so the piece's bounding box stays on the board.
+  (int, int) snapAnchor(Piece piece, Offset pieceTopLeft) {
+    var maxDx = 0;
+    var maxDy = 0;
+    for (final (dx, dy) in piece.offsets) {
+      maxDx = math.max(maxDx, dx);
+      maxDy = math.max(maxDy, dy);
+    }
+    final x = (pieceTopLeft.dx / cellSize).round();
+    final y = (pieceTopLeft.dy / cellSize).round();
+    return (
+      x.clamp(0, GridState.size - 1 - maxDx),
+      y.clamp(0, GridState.size - 1 - maxDy),
+    );
+  }
+}