feat: scaffold mvp shell and content runtime

js/gameplay/difficulty/index.js

@@ -0,0 +1,9 @@
+export { classifyDeadlock } from './classify-deadlock.js'
+export { evaluateBoard } from './evaluate-board.js'
+export { generateBoard } from './generate-board.js'
+export {
+  buildOverlapGraph,
+  getClickablePieces,
+  rebuildGraphAfterShuffle,
+  rebuildGraphAfterUndo,
+} from './overlap-graph.js'