Firmware simulation, Unity project, and sim-verified robot fixes

Simulation
- sim/: the unchanged robot firmware compiled into hexapod_sim.dll
  against PC stand-ins (Arduino, Servo, Wire + register-level MPU6050,
  RF24 with matching-config radio), a kinematic body model (support
  plane + planted-foot odometry) that also drives the simulated IMU,
  and a headless smoke test (sim_smoke). build.ps1 builds and tests.
- Hexapod_Simulation_Unity/ folded in (history kept in a bundle
  outside the repo). Assets/HexapodSim loads the DLL, drives the
  existing leg rig, and offers joystick / keyboard / robot-serial
  input plus a test panel.

Robot (all found or confirmed in the simulation)
- Config.h geometry taken from the Unity model.
- IK femur elevation sign, gait stance direction, body pose
  rotation direction, yaw lever arm.
- Stance feet integrated from the current command; gait restarts at
  mid-stance; heading integrates the applied turn rate, only while
  walking.
- Leg clearance and reach checks (Limits::feetFeasible); walk command
  and body pose scaled as a whole, pose scale rate-limited, planted
  feet guarded so the body never snaps or legs collide.
- Robot-gyro self-leveling as an integrator.
- Heading mode: steering relative to the body; S walks backward.
- Standing height from the remote (STANCE_HEIGHT_RANGE_MM).

Protocol v3 (reflash both boards)
- CmdPacket gains body_height (22 bytes).

Remote
- Height slider (Nextion hHeight/nHeight, serial [ ]).
- Fixed unspecified I2C read order in RemoteIMU.

LEGACY/: owner's old Nextion HMI and remote hardware assets.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

2026-current/Hexapod_Simulation_Unity/Assets/HexapodSim/RobotFrame.cs

@@ -0,0 +1,38 @@
+using UnityEngine;
+
+namespace HexapodSim
+{
+    // Which Unity axis the robot's forward (+X) maps to. The old scene marks
+    // the legs on Unity -Z as "invertForLeftLeg", which makes -X forward.
+    public enum ForwardAxis { NegativeX, PositiveX }
+
+    // Conversions between the firmware's body frame (right-handed, mm:
+    // +X forward, +Y left, +Z up) and Unity (left-handed, Y up). Everything
+    // physical in the simulation is computed in the robot frame and only
+    // converted here, in one place.
+    public static class RobotFrame
+    {
+        public static Vector3 ToUnity(Vector3 r, ForwardAxis f) =>
+            f == ForwardAxis.NegativeX ? new Vector3(-r.x, r.z, -r.y) : new Vector3(r.x, r.z, r.y);
+
+        public static Vector3 ToRobot(Vector3 u, ForwardAxis f) =>
+            f == ForwardAxis.NegativeX ? new Vector3(-u.x, -u.z, u.y) : new Vector3(u.x, u.z, u.y);
+
+        // A rotation expressed in the robot frame, as the equivalent Unity rotation.
+        // (The mapping is a reflection, so this is M * R * M^-1, built from two axes.)
+        public static Quaternion ToUnity(Quaternion robotRot, ForwardAxis f)
+        {
+            Vector3 fwd = ToUnity(robotRot * ToRobot(Vector3.forward, f), f);
+            Vector3 up = ToUnity(robotRot * ToRobot(Vector3.up, f), f);
+            return Quaternion.LookRotation(fwd, up);
+        }
+
+        public static float WrapDeg(float d)
+        {
+            d %= 360f;
+            if (d > 180f) d -= 360f;
+            if (d < -180f) d += 360f;
+            return d;
+        }
+    }
+}