feat(tetris-attack): smooth panel fall animation and settling frame

- Track `_prevRow` on falling panels for interpolation during animation
- Add `_justLanded` flag to introduce a settling frame after panels land
- Interpolate panel Y position from previous row for smoother fall effect
- Defer match detection and chain-end logic until after settling frame
- Update assets: new background-menu and main-title images, update existing sprites

src/games/tetrisattack/TetrisAttackGame.js

@@ -343,7 +343,14 @@ export default class TetrisAttackGame extends Phaser.Scene {
         if (spr.dying) continue;
         const pos = this.cellCenter(r, c);
         spr.x = pos.x;
-        spr.y = pos.y + (p.state === 'falling' ? fp * CELL : 0);
+        if (p.state === 'falling' && p._prevRow != null) {
+          // interpolate from previous row to current row for smooth fall animation
+          const fp = st.fallTick / TUNING.FALL_INTERVAL;
+          const prevPos = this.cellCenter(p._prevRow, c);
+          spr.y = prevPos.y + (pos.y - prevPos.y) * fp;
+        } else {
+          spr.y = pos.y + (p.state === 'falling' ? fp * CELL : 0);
+        }
         spr.setDepth(D.panels);
       }
     }

src/games/tetrisattack/TetrisAttackLogic.js

@@ -320,6 +320,7 @@ function updateFallStates(board) {
       const p = board[r][c];
       if (p && p.state === 'falling' && supported(board, r, c)) {
         p.state = 'idle';
+        delete p._prevRow;  // landed — clear animation origin
         landed.push({ r, c, id: p.id });
       }
     }
@@ -334,6 +335,7 @@ function applyFallStep(board) {
     for (let r = board.length - 2; r >= 0; r--) {
       const p = board[r][c];
       if (p && p.state === 'falling' && board[r + 1][c] === null) {
+        p._prevRow = r;  // remember old row for fall animation
         board[r + 1][c] = p;
         board[r][c] = null;
         moves.push({ col: c, fromR: r, toR: r + 1, id: p.id });
@@ -407,13 +409,20 @@ export function step(state, rng = Math.random) {
       // re-evaluate landings after the move
       const landed2 = updateFallStates(board);
       for (const l of landed2) events.push({ type: 'land', ...l });
+      if (landed2.length) state._justLanded = true;
     }
+  } else if (state._justLanded) {
+    // one settling frame after last panels land: show final animation frame
+    // before match detection fires, so the player sees panels arrive in place.
+    state.fallTick = TUNING.FALL_INTERVAL;
+    state._justLanded = false;
   } else {
     state.fallTick = 0;
   }
 
-  // 3. Match detection over settled idle panels.
-  const groups = findMatchGroups(board);
+  // 3. Match detection over settled idle panels (skip during settling frame).
+  if (!state._justLanded) {
+    const groups = findMatchGroups(board);
   if (groups.length) {
     let hadChainMember = false;
     for (const g of groups) {
@@ -450,16 +459,20 @@ export function step(state, rng = Math.random) {
     state.score += tickScore;
     events.push({ type: 'clear', cells: allCells, groups: groups.map((g) => g.size), combo: biggestCombo, chain: state.chain, score: tickScore });
   }
+  } // end: if (!state._justLanded)
 
   // 4. Wave end: board settled with no clears / falls / new matches → reset chain.
-  const settled = state.clears.length === 0 && !anyFalling(board);
-  if (settled) {
-    if (state.chain > 0) {
-      if (state.chain > 1) events.push({ type: 'chainEnd', chain: state.chain });
-      state.chain = 0;
-      state.combo = 0;
-      // clear lingering chain flags
-      for (const row of board) for (const p of row) if (p) p.chain = false;
+  //    (skip during settling frame — chain may still continue)
+  if (!state._justLanded) {
+    const settled = state.clears.length === 0 && !anyFalling(board);
+    if (settled) {
+      if (state.chain > 0) {
+        if (state.chain > 1) events.push({ type: 'chainEnd', chain: state.chain });
+        state.chain = 0;
+        state.combo = 0;
+        // clear lingering chain flags
+        for (const row of board) for (const p of row) if (p) p.chain = false;
+      }
     }
   }
 
@@ -496,7 +509,7 @@ function finishStep(state, events) {
   if (!inDanger && state.danger) { state.danger = false; events.push({ type: 'danger', on: false }); }
 
   if (!state.over && !state.won) {
-    const idleSettled = state.clears.length === 0 && !anyFalling(board);
+    const idleSettled = state.clears.length === 0 && !anyFalling(board) && !state._justLanded;
     if (state.mode === 'puzzle') {
       if (isBoardEmpty(board)) { state.won = true; events.push({ type: 'win' }); }
       else if (idleSettled && state.movesLeft <= 0) { state.over = true; events.push({ type: 'gameOver' }); }
@@ -509,7 +522,7 @@ function finishStep(state, events) {
 
 // ── Query helpers (renderer / verifier) ─────────────────────────────────────
 export function isSettled(state) {
-  return state.clears.length === 0 && !anyFalling(state.board);
+  return state.clears.length === 0 && !anyFalling(state.board) && !state._justLanded;
 }
 
 export function remainingTargets(state) {