feat: add box selection with Ctrl+drag for multi-piece manipulation

- Implement screen-to-world coordinate conversion to support camera zoom/pan
- Add Ctrl+drag box selection that highlights area and selects all pieces within
- Support snapping and releasing multiple groups independently during box release
- Track selected pieces via `_boxSelectedIds` set, including their group peers
- Adjust depth, network broadcasting, and glow rendering for multi-selection context

js/scenes/PuzzleScene.js

@@ -25,6 +25,17 @@ class PuzzleScene extends Phaser.Scene {
     super({ key: 'PuzzleScene' });
   }
 
+  /** Convert screen coordinates to world coordinates, accounting for zoom origin. */
+  _screenToWorld(screenX, screenY) {
+    const cam = this.cameras.main;
+    const ox = cam.width / 2;
+    const oy = cam.height / 2;
+    return {
+      x: (screenX - ox) / cam.zoom + cam.scrollX + ox,
+      y: (screenY - oy) / cam.zoom + cam.scrollY + oy,
+    };
+  }
+
   // ─── Phaser lifecycle ────────────────────────────────────────────────
 
   init(data) {
@@ -172,8 +183,28 @@ class PuzzleScene extends Phaser.Scene {
       this._playNextTrack();
     }
 
-    // Camera pan (right-click drag, or left-click drag on empty space)
     const ptr = this.input.activePointer;
+
+    // Box selection drawing (CTRL+drag)
+    if (this._isBoxSelecting && ptr.isDown) {
+      const curWorld = this._screenToWorld(ptr.x, ptr.y);
+      const sx = this._boxStartWorld.x;
+      const sy = this._boxStartWorld.y;
+      const ex = curWorld.x;
+      const ey = curWorld.y;
+      const rx = Math.min(sx, ex);
+      const ry = Math.min(sy, ey);
+      const rw = Math.abs(ex - sx);
+      const rh = Math.abs(ey - sy);
+
+      this._boxGraphics.clear();
+      this._boxGraphics.lineStyle(2, 0x44aaff, 0.8);
+      this._boxGraphics.fillStyle(0x44aaff, 0.12);
+      this._boxGraphics.fillRect(rx, ry, rw, rh);
+      this._boxGraphics.strokeRect(rx, ry, rw, rh);
+    }
+
+    // Camera pan (right-click drag, or left-click drag on empty space)
     if (this._isPanning && ptr.isDown && !this._heldPiece) {
       const cam = this.cameras.main;
       const pdx = (ptr.x - this._panLastX) / cam.zoom;
@@ -279,6 +310,12 @@ class PuzzleScene extends Phaser.Scene {
     this._panLastY = 0;
     this._ready = true;
 
+    // Box selection state (CTRL+drag)
+    this._isBoxSelecting = false;
+    this._boxStartWorld = null;   // { x, y } in world coords
+    this._boxGraphics = this.add.graphics().setDepth(510).setScrollFactor(1);
+    this._boxSelectedIds = null;  // Set<pieceId> when multi-holding
+
     // Input
     this.input.on('gameobjectdown', this._onPieceDown, this);
     this.input.on('pointerdown', this._onPointerDown, this);
@@ -692,6 +729,13 @@ class PuzzleScene extends Phaser.Scene {
     this._lastPtrY = ptr.y;
     this._dragDistance = 0;
 
+    // CTRL + left-click on empty space → start box selection
+    if (ptr.event && ptr.event.ctrlKey && !this._heldPiece) {
+      this._isBoxSelecting = true;
+      this._boxStartWorld = this._screenToWorld(ptr.x, ptr.y);
+      return;
+    }
+
     // Left-click on empty space (no piece held) → start camera pan
     if (!this._heldPiece) {
       this._isPanning = true;
@@ -728,6 +772,14 @@ class PuzzleScene extends Phaser.Scene {
   }
 
   _onPointerUp() {
+    // Finalize box selection
+    if (this._isBoxSelecting) {
+      this._isBoxSelecting = false;
+      this._boxGraphics.clear();
+      this._finalizeBoxSelection();
+      return;
+    }
+
     if (!this._ready || !this._heldPiece) return;
 
     if (this._justPickedUp && this._dragDistance < DRAG_THRESHOLD) {
@@ -747,6 +799,63 @@ class PuzzleScene extends Phaser.Scene {
     return null;
   }
 
+  // ─── Box selection ───────────────────────────────────────────────────
+
+  _finalizeBoxSelection() {
+    const ptr = this.input.activePointer;
+    const endWorld = this._screenToWorld(ptr.x, ptr.y);
+    const sx = this._boxStartWorld.x;
+    const sy = this._boxStartWorld.y;
+    const minX = Math.min(sx, endWorld.x);
+    const minY = Math.min(sy, endWorld.y);
+    const maxX = Math.max(sx, endWorld.x);
+    const maxY = Math.max(sy, endWorld.y);
+
+    // Too small a box — ignore
+    if ((maxX - minX) < 5 && (maxY - minY) < 5) return;
+
+    // Find all pieces whose centers fall within the box
+    const selectedIds = new Set();
+    this._pieceObjects.forEach((po, id) => {
+      if (this._remoteClaims.has(id)) return; // skip pieces held by other players
+      const px = po.data.x;
+      const py = po.data.y;
+      if (px >= minX && px <= maxX && py >= minY && py <= maxY) {
+        // Add this piece and all its group peers
+        const peers = this._groupManager.getPeersOf(id);
+        peers.forEach(pid => selectedIds.add(pid));
+      }
+    });
+
+    if (selectedIds.size === 0) return;
+
+    // Pick the first piece as the anchor for the held-piece system
+    const anchorId = selectedIds.values().next().value;
+    const anchorPO = this._pieceObjects.get(anchorId);
+
+    this._boxSelectedIds = selectedIds;
+    this._heldPiece = anchorPO;
+    this._dragDistance = 0;
+    this._justPickedUp = true;
+    this.sound.play('sfx_grab', { volume: 0.3 });
+
+    const ptrNow = this.input.activePointer;
+    this._lastPtrX = ptrNow.x;
+    this._lastPtrY = ptrNow.y;
+
+    // Raise depth of all selected pieces
+    let depthOffset = 0;
+    selectedIds.forEach(id => {
+      this._pieceObjects.get(id).setDepth(600 + depthOffset);
+      depthOffset++;
+    });
+
+    // Network: claim the anchor piece
+    if (this._isNetworked) {
+      NetworkManager.claimPiece(anchorPO.data.id);
+    }
+  }
+
   // ─── Pick-up / release ───────────────────────────────────────────────
 
   _pickUp(pieceObj) {
@@ -780,6 +889,57 @@ class PuzzleScene extends Phaser.Scene {
 
     const releasedPieceId = this._heldPiece.data.id;
 
+    if (this._boxSelectedIds) {
+      // Box selection release: snap and reset each distinct group independently
+      const processedGroups = new Set();
+
+      if (trySnap) {
+        // Collect distinct groups from the selection
+        const groupAnchors = [];
+        this._boxSelectedIds.forEach(id => {
+          const groupId = this._groupManager.getGroupId(id);
+          if (!processedGroups.has(groupId)) {
+            processedGroups.add(groupId);
+            groupAnchors.push(id);
+          }
+        });
+
+        // Snap each group independently
+        groupAnchors.forEach(anchorId => {
+          const fakePiece = this._pieceObjects.get(anchorId);
+          const saved = this._heldPiece;
+          this._heldPiece = fakePiece;
+          this._doSnap();
+          this._heldPiece = saved;
+        });
+      }
+
+      // Reset depth for all selected pieces
+      this._boxSelectedIds.forEach(id => {
+        this._pieceObjects.get(id).setDepth(0);
+      });
+
+      // Broadcast release for each distinct group
+      if (this._isNetworked) {
+        const sentGroups = new Set();
+        this._boxSelectedIds.forEach(id => {
+          const groupId = this._groupManager.getGroupId(id);
+          if (!sentGroups.has(groupId)) {
+            sentGroups.add(groupId);
+            const peers = this._groupManager.getPeersOf(id);
+            const positions = [];
+            peers.forEach(pid => {
+              const po = this._pieceObjects.get(pid);
+              positions.push({ id: pid, x: po.data.x, y: po.data.y });
+            });
+            NetworkManager.sendRelease(id, positions, this._groupManager.serialize());
+          }
+        });
+      }
+
+      this._boxSelectedIds = null;
+    } else {
+      // Normal single-piece/group release
       if (trySnap) {
         this._doSnap();
       }
@@ -801,6 +961,7 @@ class PuzzleScene extends Phaser.Scene {
           this._groupManager.serialize()
         );
       }
+    }
 
     this._glowGraphics.clear();
     this._heldPiece = null;
@@ -813,8 +974,9 @@ class PuzzleScene extends Phaser.Scene {
   }
 
   _moveHeldGroup(dx, dy) {
-    const peers = this._groupManager.getPeersOf(this._heldPiece.data.id);
+    // If box-selected, move all selected pieces; otherwise just the held group
-    peers.forEach(id => {
+    const ids = this._boxSelectedIds || this._groupManager.getPeersOf(this._heldPiece.data.id);
+    ids.forEach(id => {
       const po = this._pieceObjects.get(id);
       po.setPosition(po.data.x + dx, po.data.y + dy);
     });
@@ -884,7 +1046,7 @@ class PuzzleScene extends Phaser.Scene {
 
   _drawGlow() {
     this._glowGraphics.clear();
-    if (!this._heldPiece) return;
+    if (!this._heldPiece || this._boxSelectedIds) return;
 
     const glowRadius = this._pieceW * GLOW_RADIUS_FACTOR;
     const segments = SnapDetector.getGlowSegments(