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/robot/Limits.h

@@ -5,6 +5,17 @@ class Limits {
 public:
   // Clamps a foot target into a conservative safe envelope around the leg's neutral stance.
   static FootTarget clampWorkspace(int leg_index, const FootTarget& target);
+  // True if clampWorkspace() would leave the target unchanged.
+  static bool inWorkspace(int leg_index, const FootTarget& target);
+
+  // True if all six feet can be placed there at once: each target in its
+  // workspace with a valid IK solution, and no two neighbouring legs'
+  // segments closer than LEG_CLEARANCE_MM.
+  static bool feetFeasible(const FootTarget feet[6]);
+
+  // Smallest distance between neighbouring legs' segments for these feet
+  // (diagnostics; a large negative number if some leg has no IK solution).
+  static float legClearance(const FootTarget feet[6]);
 
   // Rate-limits a joint angle change to the servo's real max speed.
   static float clampJointRate(float current_deg, float target_deg, float max_deg_per_sec, float dt);