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/remote/RemoteIMU.cpp

@@ -55,8 +55,12 @@ bool RemoteIMU::readRaw(float a[3], float g[3]) {
   Wire.requestFrom((int)IMU_ADDR, 14);
   if (Wire.available() < 14) return false;
 
-  int16_t raw[7];
-  for (int k = 0; k < 7; k++) raw[k] = (Wire.read() << 8) | Wire.read();   // ax ay az temp gx gy gz
+  int16_t raw[7];   // ax ay az temp gx gy gz, big-endian
+  for (int k = 0; k < 7; k++) {
+    uint8_t hi = Wire.read();   // two statements: the order of calls inside one expression is unspecified
+    uint8_t lo = Wire.read();
+    raw[k] = (int16_t)((hi << 8) | lo);
+  }
 
   float chip_a[3] = { raw[0] / 16384.0f, raw[1] / 16384.0f, raw[2] / 16384.0f };
   float chip_g[3] = { raw[4] / 131.0f,   raw[5] / 131.0f,   raw[6] / 131.0f };