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/sim/robot_ino.cpp

@@ -0,0 +1,22 @@
+// Compiles robot/robot.ino unchanged as part of the simulation, then exposes
+// read-only views of its globals so the simulation can report robot state
+// without the robot code having to know it's being simulated.
+#include "../robot/robot.ino"
+#include "SimRobotAccess.h"
+
+namespace simrobot {
+void setupOnce()                 { setup(); }
+void loopOnce()                  { loop(); }
+int state()                      { return (int)state_machine.state(); }
+float sitStandBlend()            { return state_machine.sitStandBlend(); }
+bool linkOk()                    { return comms.linkOk(); }
+float headingDeg()               { return current_heading_deg; }
+float imuRollDeg()               { return imu.rollDeg(); }
+float imuPitchDeg()              { return imu.pitchDeg(); }
+bool imuEnabled()                { return imu.enabled; }
+uint32_t lastLoopMicros()        { return last_loop_us; }
+bool legInStance(int leg)        { return gait.legInStance(leg); }
+float poseReachScale()           { return pose.reachScale(); }
+float twistScale()               { return gait.twistScale(); }
+float stanceHeightMm()           { return Kinematics::stanceHeight(); }
+}  // namespace simrobot