Add observation_function for full custom control over the observation

Lets a game define how its state becomes an observation as a plain
Python function (module:attr), used identically by GameEnvironment
(training) and TrainedPolicy (inference) instead of two independently
maintained encoding paths. Removes the egocentric/egocentric_player/
egocentric_radius flags — cropping is now something an
observation_function does itself by calling egocentric_board(), and
extras_size is discovered from one sampled observation instead of
being configured via observe_state_sizes.

retro_gamer/observation.py

@@ -64,25 +64,24 @@ def encode_observation(
     state: dict,
     metadata: GameMetadata,
     observe_state: list[str],
-    player_pos: tuple[int, int] | None = None,
-    egocentric_radius: int | None = None,
     board: bool = True,
 ) -> np.ndarray:
     """Encode board and/or selected state values into a flat 1D observation vector.
 
-    When *board* is True the board is encoded and prepended to the vector. If
-    player_pos and egocentric_radius are given the board is first cropped to a
-    (2r+1)×(2r+1) window centred on the player. For spatial games the board is
-    encoded channel-first (C, H, W) then flattened; for non-spatial games it is
-    encoded (H, W, C) then flattened. The state vector is appended at the end.
+    When *board* is True the board is encoded and prepended to the vector. For
+    spatial games the board is encoded channel-first (C, H, W) then flattened;
+    for non-spatial games it is encoded (H, W, C) then flattened. The state
+    vector is appended at the end.
 
     When *board* is False only the observe_state features are returned.
+
+    For cropped (egocentric) boards, write a custom observation_function that
+    calls egocentric_board() and encode_board() directly instead of using this
+    function.
     """
     if board:
         if not metadata.character_set:
             raise ValueError("character_set must be set before encoding observations")
-        if player_pos is not None and egocentric_radius is not None:
-            board_chars = egocentric_board(board_chars, player_pos, egocentric_radius)
         board_enc = encode_board(board_chars, metadata.character_set)  # (H, W, C)
         if metadata.spatial:
             board_vec = board_enc.transpose(2, 0, 1).flatten()