feat(morphology): support per-arm segment counts for partial amputations

configs/example.yaml

@@ -1,5 +1,4 @@
 morphology:
-  num_arms: 2
+  segments_per_arm: [4, 4]
-  num_segments_per_arm: 4
   use_p_control: true
   use_torque_control: false

src/brittle_star_project/environment/env_config.py

@@ -7,11 +7,24 @@ from .env_types import Task
 
 @dataclass(frozen=True, slots=True)
 class MorphologyConfig:
-    num_arms: int = 5
+    """Brittle star morphology configuration.
-    num_segments_per_arm: int = 4
+
+    segments_per_arm defines the number of segments for each arm. The length of
+    this list implicitly sets the number of arms. Use 0 segments to represent
+    a fully amputated arm (e.g., [4, 0, 4, 2, 4] for a 5-arm morphology with
+    arm 1 removed and arm 3 shortened).
+
+    The upstream biorobot library natively supports per-arm segment counts.
+    """
+
+    segments_per_arm: tuple[int, ...] = (4, 4, 4, 4, 4)
     use_p_control: bool = True
     use_torque_control: bool = False
 
+    @property
+    def num_arms(self) -> int:
+        return len(self.segments_per_arm)
+
 
 @dataclass(frozen=True, slots=True)
 class ArenaConfig:

src/brittle_star_project/environment/factory.py

@@ -22,7 +22,7 @@ class BrittleStarEnvFactory:
 
         spec = default_brittle_star_morphology_specification(
             num_arms=config.num_arms,
-            num_segments_per_arm=config.num_segments_per_arm,
+            num_segments_per_arm=list(config.segments_per_arm),
             use_p_control=config.use_p_control,
             use_torque_control=config.use_torque_control,
         )