Implement DAS/ARR for smooth horizontal piece movement

CLAUDE.md

@@ -79,14 +79,39 @@ This design gives players a buffer zone to recover from garbage attacks before e
 | `player-move` | Client->Server | `{ playerId, direction }` |
 | `player-rotate` | Client->Server | `{ playerId }` |
 | `player-drop` | Client->Server | `{ playerId, hard }` |
+| `player-hold` | Client->Server | `{ playerId }` |
+| `zone-activate` | Client->Server | `{ playerId }` |
 | `player-joined` | Server->Client | `{ player, players }` |
 | `player-left` | Server->Client | `{ playerId, players }` |
 | `game-started` | Server->Client | `{ players, states }` |
 | `state-update` | Server->Client | `states[]` |
 | `game-over` | Server->Client | `{ states }` |
 
+### Zone Mechanic (Tetris Effect)
+
+Zone is a time-based attack mechanic imported from Tetris Effect. When players clear lines, their **Zone meter** fills:
+- **1 line** = +15 Zone meter
+- **2 lines** = +25 Zone meter
+- **3 lines** = +35 Zone meter
+- **4 lines (Tetris)** = +50 Zone meter
+
+**When Zone meter reaches 100:**
+- Press **Z key** or **ZONE button** to activate Zone
+- Gravity pauses for **20 seconds**
+- Players control pieces manually but gravity is frozen
+- Cleared lines **cascade to bottom** instead of disappearing
+- After 20 seconds, Zone ends and sends garbage to opponents
+
+**Zone Attack Calculation:**
+- Lines cleared during Zone × 1.5 = garbage rows sent
+- Example: 4 Zone lines = 6 garbage rows, 8 Zone lines = 12 garbage rows
+- Garbage is distributed randomly among remaining opponents
+
+Zone provides a high-risk, high-reward tactic for eliminating opponents with massive attacks.
+
 ### Rendering
 
 - Each player gets a dynamically created board with canvas + info divs
 - `renderer.setActivePlayer()` marks current player's board as `.main`, others as `.spectator`
+- Zone meter shown below each player board with fill bar and status indicator
 - Battle grid layout classes: `.single-player`, `.two-players`, `.multi-player`