Add pure-Dart engine core: RNG, grid, placement, line clear, scoring, piece generator

PCG32 seeded RNG; immutable 8x8 GridState with occupancy bitmask;
placement legality + anyPlacementExists; simultaneous row/col clears
with single-count gem credit; combo scoring with one-move grace;
weighted-bag generator with pity bias and depth-3 solvability nudge.
All TDD, 51 tests green.

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

lib/game/engine/placement.dart

@@ -0,0 +1,41 @@
+import '../models/cell.dart';
+import '../models/grid.dart';
+import '../models/piece.dart';
+
+/// Whether [piece] fits with its anchor at ([x], [y]).
+bool canPlace(GridState grid, Piece piece, int x, int y) {
+  for (final (dx, dy) in piece.offsets) {
+    final px = x + dx;
+    final py = y + dy;
+    if (px < 0 || px >= GridState.size || py < 0 || py >= GridState.size) {
+      return false;
+    }
+    if (grid.isOccupied(px, py)) return false;
+  }
+  return true;
+}
+
+/// Places [piece] at ([x], [y]); caller must check [canPlace] first.
+GridState place(GridState grid, Piece piece, int x, int y) {
+  var next = grid;
+  for (final (dx, dy) in piece.offsets) {
+    next = next.withCell(
+      x + dx,
+      y + dy,
+      Cell(CellType.filled, colorId: piece.colorId),
+    );
+  }
+  return next;
+}
+
+/// Whether at least one of [pieces] has at least one legal placement.
+bool anyPlacementExists(GridState grid, List<Piece> pieces) {
+  for (final piece in pieces) {
+    for (var y = 0; y < GridState.size; y++) {
+      for (var x = 0; x < GridState.size; x++) {
+        if (canPlace(grid, piece, x, y)) return true;
+      }
+    }
+  }
+  return false;
+}