✨ feat(esphome): add head joints JSON sensor for 3D visualization

- add get_head_joints_json() method to ReachyController
- add Phase 13 entity: head_joints (single JSON sensor)
- returns [yaw_body, stewart_1, ..., stewart_6] array in radians
- replaces complex multi-entity approach with single JSON sensor
- improves 3D model pose synchronization in HA dashboard

This allows the Home Assistant 3D card to properly control
all 7 head joints using the URDF joint system, matching
the desktop-app implementation.

reachy_mini_ha_voice/reachy_controller.py

@@ -1006,3 +1006,27 @@ class ReachyController:
         except Exception as e:
             logger.debug(f"Error getting AEC converged status: {e}")
         return False
+
+    # ========== Phase 13: Robot Joints ==========
+
+    def get_head_joints_json(self) -> str:
+        """
+        Get head joints as JSON string.
+
+        Returns:
+            JSON string: "[yaw_body, stewart_1, stewart_2, stewart_3, stewart_4, stewart_5, stewart_6]"
+            Values in radians
+        """
+        if not self.is_available:
+            return "[]"
+        try:
+            import json
+            head_joints, _ = self.reachy.get_current_joint_positions()
+            if head_joints and len(head_joints) >= 7:
+                # Convert radians to list
+                joints_list = [float(j) for j in head_joints[:7]]
+                return json.dumps(joints_list)
+            return "[]"
+        except Exception as e:
+            logger.error(f"Error getting head joints JSON: {e}")
+            return "[]"

reachy_mini_ha_voice/satellite.py

@@ -126,6 +126,8 @@ class VoiceSatelliteProtocol(APIServer):
         "agc_max_gain": 1201,
         "noise_suppression": 1202,
         "echo_cancellation_converged": 1203,
+        # Phase 13: Robot joints (single JSON sensor)
+        "head_joints": 1300,
     }
 
     def _get_entity_key(self, object_id: str) -> int:
@@ -176,6 +178,7 @@ class VoiceSatelliteProtocol(APIServer):
             self._setup_phase10_entities()  # Camera
             # Phase 11 (LED control) disabled - LEDs are inside the robot and not visible
             self._setup_phase12_entities()  # Audio processing
+            self._setup_phase13_entities()  # Robot joints
 
             # Mark entities as initialized
             self.state._entities_initialized = True
@@ -1413,3 +1416,19 @@ class VoiceSatelliteProtocol(APIServer):
         self.state.entities.append(echo_cancellation_converged)
 
         _LOGGER.info("Phase 12 entities registered: agc_enabled, agc_max_gain, noise_suppression, echo_cancellation_converged")
+
+    def _setup_phase13_entities(self) -> None:
+        """Setup Phase 13 entities: Robot joints as JSON sensor."""
+
+        # Head joints sensor - returns JSON array of [yaw_body, stewart_1, ..., stewart_6]
+        head_joints_sensor = TextSensorEntity(
+            server=self,
+            key=self._get_entity_key("head_joints"),
+            name="Head Joints",
+            object_id="head_joints",
+            icon="mdi:robot",
+            value_getter=self.reachy_controller.get_head_joints_json,
+        )
+        self.state.entities.append(head_joints_sensor)
+
+        _LOGGER.info("Phase 13 entities registered: head_joints")