Initial import: robot firmware + PlatformIO two-env scaffold

Robot-side firmware (Teensy 4.0) imported unchanged as extracted.
platformio.ini defines [env:robot] (teensy40) and [env:remote]
(teensy41); scripts/select_src_dir.py points each env at its own
sketch folder so robot/ and remote/ stay Arduino-style sketches.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

2026-current/robot/GaitEngine.h

@@ -0,0 +1,35 @@
+#pragma once
+#include "Kinematics.h"
+#include "Config.h"
+
+struct GaitPattern {
+  float duty_factor;              // fraction of the cycle each leg spends in stance
+  float leg_phase_offset[6];      // each leg's phase offset within the cycle, 0..1
+};
+
+extern const GaitPattern GAIT_TRIPOD;
+extern const GaitPattern GAIT_WAVE;
+
+struct Twist {
+  float vx = 0, vy = 0;   // mm/s, body frame
+  float omega = 0;        // rad/s, body frame
+};
+
+class GaitEngine {
+public:
+  void begin();
+  void setRequestedPattern(const GaitPattern* p) { requested_pattern_ = p; }
+  void setCycleDuration(float sec) { cycle_sec_ = sec; }
+
+  // Advances every leg by dt under the current twist and pattern, writing each
+  // leg's target foot position (body frame) into out[6].
+  void update(float dt, const Twist& twist, FootTarget out[6]);
+
+  bool legInStance(int i) const { return leg_phase_[i] < legs_pattern_[i]->duty_factor; }
+
+private:
+  float leg_phase_[6];
+  const GaitPattern* legs_pattern_[6];
+  const GaitPattern* requested_pattern_ = &GAIT_TRIPOD;
+  float cycle_sec_ = DEFAULT_CYCLE_SEC;
+};