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/README.md

@@ -36,6 +36,13 @@ Stick pots are wired 3.3 V – wiper – GND. **Never feed 5 V into a Teensy
 | Right stick, **ROTATE** (default) | `rx/ry/rz` | body roll / pitch / yaw |
 | Right stick, **MOVE** (`FLAG_RSTICK_XLATE`) | `rx/ry/rz` | body X / Y / Z (height, via twist) offset |
 | Handset tilt | `remote_roll/pitch` | extra body roll/pitch while REMOTE GYRO is on |
+| Height slider | `body_height` | standing height, default ± `STANCE_HEIGHT_RANGE_MM` (robot rate-limits the change) |
+
+In **HEADING** mode the left stick steers, relative to where the robot
+faces at that moment. Forward/back walks forward/back, and sideways
+steers: forward+left or back+left walk while turning left, and left alone
+turns on the spot for as long as you hold it. Let go and the robot holds
+its heading.
 
 Touchscreen: ARM / DISARM, SIT/STAND, TRIPOD/WAVE, DIRECT/HEADING,
 R: ROTATE/MOVE, REMOTE GYRO, ROBOT GYRO, speed slider, LEVEL (re-zero the
@@ -91,6 +98,7 @@ display attached.
 | `m` | direct ↔ heading-hold | `t` | right stick rotate ↔ translate |
 | `r` | remote gyro on/off | `b` | robot gyro on/off |
 | `+` / `-` | speed ±25 | `z` | zero handset level |
+| `]` / `[` | height up / down 10% | | |
 | `c` | start / save calibration | `x` | cancel calibration |
 | `p` | print status (raw + mapped sticks, IMU, link, state, cal) | `d` | toggle 10 Hz packet dump |
 | `R` | reset stick cal to Config defaults | | |