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/scoring.dart

@@ -0,0 +1,50 @@
+/// Pure scoring math: placement points, line-clear bonuses, combo streaks.
+library;
+
+/// Base score for clearing [lines] rows/columns simultaneously.
+/// 1 -> 100, 2 -> 300, 3 -> 600, 4 -> 1000: simultaneous clears are
+/// superlinearly rewarded.
+int lineClearBase(int lines) => 100 * lines + 50 * (lines - 1) * lines;
+
+/// Multiplier applied to clear score at combo [streak], capped at streak 8
+/// (x5.0) so late-game scores stay bounded.
+double comboMultiplier(int streak) => 1 + 0.5 * (streak > 8 ? 8 : streak);
+
+/// Combo streak with one dry-move grace before reset.
+class ComboState {
+  const ComboState({required this.streak, required this.dryMoves});
+
+  static const initial = ComboState(streak: 0, dryMoves: 0);
+
+  final int streak;
+  final int dryMoves;
+
+  /// One dry move keeps the streak alive (grace); the second resets it.
+  ComboState advance({required bool cleared}) {
+    if (cleared) return ComboState(streak: streak + 1, dryMoves: 0);
+    if (dryMoves + 1 >= 2) return initial;
+    return ComboState(streak: streak, dryMoves: dryMoves + 1);
+  }
+}
+
+class ScoreDelta {
+  const ScoreDelta({required this.points, required this.combo});
+
+  final int points;
+  final ComboState combo;
+}
+
+/// Computes the score delta for one placement and the resulting combo state.
+ScoreDelta scorePlacement({
+  required int cellsPlaced,
+  required int linesCleared,
+  required ComboState combo,
+}) {
+  final next = combo.advance(cleared: linesCleared > 0);
+  var points = cellsPlaced;
+  if (linesCleared > 0) {
+    points += (lineClearBase(linesCleared) * comboMultiplier(next.streak))
+        .round();
+  }
+  return ScoreDelta(points: points, combo: next);
+}