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/Kinematics.h

@@ -0,0 +1,22 @@
+#pragma once
+#include <Arduino.h>
+
+struct JointAngles {
+  float coxa_deg;
+  float femur_deg;
+  float tibia_deg;
+  bool valid;   // false if the target was unreachable or outside joint limits
+};
+
+struct FootTarget {
+  float x, y, z;   // body-frame mm, relative to body center
+};
+
+class Kinematics {
+public:
+  // Solves leg i's joint angles for a body-frame foot target.
+  static JointAngles solve(int leg_index, const FootTarget& target);
+
+  // The leg's resting/neutral foot target, in body frame.
+  static FootTarget neutralStance(int leg_index);
+};