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/HexapodSimApi.h

@@ -0,0 +1,90 @@
+#pragma once
+// C API of hexapod_sim.dll: the unchanged robot firmware running against
+// simulated hardware. Consumed by Unity (Assets/HexapodSim/HexapodSimNative.cs)
+// and by the headless test (tests/sim_smoke.cpp).
+//
+// Conventions: robot body frame, mm and degrees - +X forward, +Y left, +Z up
+// (same as shared/HexapodProtocol). Per-leg arrays are [leg][joint or xyz]
+// flattened, leg order as in robot/Config.h, joints coxa/femur/tibia.
+//
+// One robot per loaded DLL: its globals are the firmware's globals. To reset
+// the robot, unload and reload the DLL (Unity does this on every Play).
+#include <stdint.h>
+
+#ifdef _WIN32
+#define HXSIM_API extern "C" __declspec(dllexport)
+#else
+#define HXSIM_API extern "C" __attribute__((visibility("default")))
+#endif
+
+// Bump whenever a signature or struct below changes; the C# side checks it.
+#define HXSIM_API_VERSION 5
+
+#define HXSIM_NUM_LEGS 6
+
+// All fields 4 bytes wide so C# can mirror this with LayoutKind.Sequential.
+typedef struct {
+  int32_t num_legs;
+  float mount_x_mm[HXSIM_NUM_LEGS];
+  float mount_y_mm[HXSIM_NUM_LEGS];
+  float mount_angle_deg[HXSIM_NUM_LEGS];
+  float coxa_mm, femur_mm, tibia_mm;
+  float stance_radius_mm, stance_height_mm;
+  float coxa_min_deg, coxa_max_deg;
+  float femur_min_deg, femur_max_deg;
+  float tibia_min_deg, tibia_max_deg;
+  float control_hz;
+  int32_t servo_us_at_0deg;
+  int32_t servo_us_per_deg;
+  float leg_clearance_mm;    // LEG_CLEARANCE_MM: minimum gap between neighbouring legs' segments
+  float stance_height_range_mm;   // CmdPacket.body_height +-1000 moves the height by this much
+} HxGeometry;
+
+typedef struct {
+  int32_t robot_state;       // 0 DISARMED, 1 SIT, 2 STAND, 3 WALKING
+  float sit_stand_blend;     // 0 sit .. 1 stand
+  int32_t link_ok;           // Comms::linkOk()
+  float heading_deg;         // robot.ino's open-loop heading estimate
+  float imu_roll_deg;        // RobotIMU filter output
+  float imu_pitch_deg;
+  int32_t imu_enabled;       // robot-gyro toggle as the robot sees it
+  uint32_t sim_time_ms;
+  uint32_t control_ticks;    // loop() passes that ran the control step
+  int32_t stance_mask;       // bit i = leg i in stance per the gait engine
+  float pose_reach_scale;    // 1 = full commanded body pose; less = scaled down to what the legs can reach
+  float twist_scale;         // 1 = walk command as sent; less = slowed to a stride the legs can take
+  float stance_height_mm;    // current (rate-limited) standing height: feet below hips
+} HxStatus;
+
+HXSIM_API int32_t hxsim_api_version(void);
+HXSIM_API int32_t hxsim_packet_size(void);          // sizeof(CmdPacket) as compiled
+HXSIM_API void    hxsim_get_geometry(HxGeometry* out);
+
+// Hardware present at power-on. Call before hxsim_start().
+HXSIM_API void    hxsim_set_radio_present(int32_t present);
+HXSIM_API void    hxsim_set_imu_present(int32_t present);
+
+HXSIM_API void    hxsim_start(void);                // runs setup() once
+// Advances simulated time by dt_us, calling loop() every HXSIM_LOOP_PERIOD_US
+// like the real busy loop (loop() itself holds the CONTROL_HZ rate).
+HXSIM_API void    hxsim_step(uint32_t dt_us);
+
+// Inputs
+HXSIM_API void    hxsim_radio_send(const uint8_t* bytes, int32_t len);   // one packet on air
+HXSIM_API void    hxsim_set_ground_tilt(float roll_deg, float pitch_deg); // tilt the world (robot-gyro tests)
+// The MPU6050 is normally fed from the kinematic body model (SimBody.cpp).
+// Calling this overrides it with fixed readings from then on.
+HXSIM_API void    hxsim_set_imu(float ax_g, float ay_g, float az_g,
+                                float gx_dps, float gy_dps, float gz_dps);
+HXSIM_API void    hxsim_serial_input(const char* bytes, int32_t len);    // TEST_SERIAL_INPUT keys
+
+// Outputs
+HXSIM_API int32_t hxsim_serial_output(char* out, int32_t cap);           // drains robot Serial prints
+HXSIM_API void    hxsim_get_servos(int32_t* us_18, int32_t* attached_18, uint32_t* writes_18);
+HXSIM_API void    hxsim_get_joints_deg(float* deg_18);   // servo pulses decoded with Config.h constants
+HXSIM_API void    hxsim_get_feet_mm(float* xyz_18);      // forward kinematics of those angles, body frame
+HXSIM_API void    hxsim_get_status(HxStatus* out);
+// Kinematic body (SimBody.cpp): the body rests on the support plane of its
+// feet and grounded feet don't slide. World frame = robot frame at power-on,
+// ground at z = 0. Any pointer may be null.
+HXSIM_API void    hxsim_get_body(float* pos_mm_3, float* quat_wxyz_4, float* yaw_deg, int32_t* contact_mask);