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/shared/HexapodProtocol/HexapodProtocol.h

@@ -28,21 +28,26 @@ struct CmdPacket {
   int16_t rx = 0, ry = 0, rz = 0;  // right stick: roll/pitch/yaw, or X/Y/Z offset if FLAG_RSTICK_XLATE
   int16_t remote_roll = 0;         // remote handset tilt (feedforward), only honored with FLAG_REMOTE_GYRO
   int16_t remote_pitch = 0;
+  int16_t body_height = 0;         // standing height setting: -1000 lowest, 0 = default, +1000 highest
   uint8_t speed_scale = 128;       // 0-255, multiplies vx/vy
   uint8_t flags = 0;               // bit-packed, see FLAG_* below
   uint8_t seq = 0;                 // increments every transmitted packet
 };
 
 // Both ends are Teensy (Cortex-M7, little-endian, 2-byte int16 alignment):
-// 8 x int16 + 3 x uint8 = 19 bytes, padded to 20. If a different MCU family
+// 9 x int16 + 3 x uint8 = 21 bytes, padded to 22. If a different MCU family
 // ever joins, pack the struct explicitly rather than trusting this.
-static_assert(sizeof(CmdPacket) == 20, "CmdPacket layout changed - reflash both boards");
+static_assert(sizeof(CmdPacket) == 22, "CmdPacket layout changed - reflash both boards");
 static_assert(sizeof(CmdPacket) <= 32, "nRF24L01 payloads are limited to 32 bytes");
 
 constexpr uint8_t FLAG_ARMED        = 1 << 0;
 constexpr uint8_t FLAG_STAND        = 1 << 1;   // 0 = sit requested, 1 = stand requested
 constexpr uint8_t FLAG_WALK         = 1 << 2;   // left stick past deadzone -> walking
-constexpr uint8_t FLAG_MODE_B       = 1 << 3;   // 0 = mode A direct, 1 = mode B heading-hold
+// Mode B (steering), relative to where the body faces right now: the left
+// stick's forward/back component walks forward/back, its sideways component
+// steers - forward+left or back+left walk while turning left, left alone
+// turns on the spot while held. Released, the robot holds its heading.
+constexpr uint8_t FLAG_MODE_B       = 1 << 3;   // 0 = mode A direct, 1 = mode B heading
 constexpr uint8_t FLAG_RSTICK_XLATE = 1 << 4;   // 0 = right stick = rotation, 1 = translation
 constexpr uint8_t FLAG_GAIT_WAVE    = 1 << 5;   // 0 = tripod requested, 1 = wave requested
 constexpr uint8_t FLAG_REMOTE_GYRO  = 1 << 6;   // remote tilt feedforward enabled