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/shim/RF24.h

@@ -0,0 +1,38 @@
+#pragma once
+#include <Arduino.h>
+
+// Stand-in for the TMRh20 RF24 library. Enum values match RF24 1.x so the
+// static_asserts in Comms.cpp still check HexapodProtocol.h.
+enum rf24_pa_dbm_e { RF24_PA_MIN = 0, RF24_PA_LOW, RF24_PA_HIGH, RF24_PA_MAX, RF24_PA_ERROR };
+enum rf24_datarate_e { RF24_1MBPS = 0, RF24_2MBPS, RF24_250KBPS };
+
+// A simulated radio. Packets the simulation puts "on air" (hxsim_radio_send)
+// are only received if this radio's channel, data rate, address and static
+// payload size match the sender's, just like the real nRF24L01. The RX FIFO
+// holds 3 packets; extra packets are dropped.
+class RF24 {
+public:
+  RF24(uint16_t ce_pin, uint16_t csn_pin) { (void)ce_pin; (void)csn_pin; }
+  bool begin();
+  bool isChipConnected();
+  void setPayloadSize(uint8_t size) { payload_size_ = size > 32 ? 32 : size; }
+  void openReadingPipe(uint8_t pipe, const uint8_t* address);
+  void openWritingPipe(const uint8_t*) {}
+  void setPALevel(rf24_pa_dbm_e, bool = true) {}
+  bool setDataRate(rf24_datarate_e rate) { data_rate_ = rate; return true; }
+  void setChannel(uint8_t ch) { channel_ = ch > 125 ? 125 : ch; }
+  void setRetries(uint8_t, uint8_t) {}
+  void startListening() { listening_ = true; }
+  void stopListening() { listening_ = false; }
+  bool available();
+  void read(void* buf, uint8_t len);
+  bool write(const void*, uint8_t) { return false; }   // the robot never transmits
+
+private:
+  uint8_t payload_size_ = 32;
+  uint8_t channel_ = 76;
+  rf24_datarate_e data_rate_ = RF24_1MBPS;
+  uint8_t address_[5] = {};
+  bool pipe_open_ = false;
+  bool listening_ = false;
+};