Server: per-tick simulation skeleton (inputs, movement, blocking/shrink, apple growth, wrap behavior) and basic state delta broadcast

- Input buffer rules: enqueue+consume with 180° guard (len>1)
- Movement: wrap per config; allow moving into own tail when it vacates
- Blocking: head holds, tail shrinks to min 1
- Apples: eat= grow; ensure target apples after tick
- Broadcast: send current snakes/apples as delta (placeholder for real deltas)

server/model.py

@@ -16,11 +16,33 @@ class Snake:
     head: Coord
     direction: Direction
     body: Deque[Coord] = field(default_factory=deque)  # includes head at index 0
+    input_buf: Deque[Direction] = field(default_factory=deque)
+    blocked: bool = False
 
     @property
     def length(self) -> int:
         return len(self.body)
 
+    def enqueue_direction(self, new_dir: Direction, capacity: int = 3) -> None:
+        """Apply input buffer rules: size<=capacity, replace last on overflow/opposite, drop duplicates."""
+        last_dir = self.input_buf[-1] if self.input_buf else self.direction
+        # Drop duplicates
+        if int(new_dir) == int(last_dir):
+            return
+        # Opposite of last? replace last
+        if (int(new_dir) ^ int(last_dir)) == 2:  # 0^2,1^3,2^0,3^1 are opposites
+            if self.input_buf:
+                self.input_buf[-1] = new_dir
+            else:
+                # No buffered inputs; just add new_dir (consumption will handle 180° rule)
+                self.input_buf.append(new_dir)
+            return
+        # Normal append with overflow replacement
+        if len(self.input_buf) >= capacity:
+            self.input_buf[-1] = new_dir
+        else:
+            self.input_buf.append(new_dir)
+
 
 @dataclass
 class PlayerSession: