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/Comms.h

@@ -0,0 +1,40 @@
+#pragma once
+#include <Arduino.h>
+
+// Remote -> robot, sent at a fixed rate. Kept as the single source of truth
+// here so a future remote sketch just has to fill this same struct and
+// radio.write() it - nothing on the robot side needs to change.
+struct CmdPacket {
+  int16_t vx = 0, vy = 0;           // left stick: strafe / forward-back, -1000..1000
+  int16_t yaw_rate = 0;             // left stick twist, -1000..1000
+  int16_t rx = 0, ry = 0, rz = 0;   // right stick: roll/pitch/yaw OR X/Y/height, per flags
+  uint8_t speed_scale = 128;        // 0-255
+  uint8_t flags = 0;                // bit-packed, see FLAG_* below
+  uint8_t seq = 0;
+};
+
+constexpr uint8_t FLAG_ARMED        = 1 << 0;
+constexpr uint8_t FLAG_STAND        = 1 << 1;   // 0 = sit requested, 1 = stand requested
+constexpr uint8_t FLAG_WALK         = 1 << 2;   // stick past deadzone -> walking
+constexpr uint8_t FLAG_MODE_B       = 1 << 3;   // 0 = mode A direct, 1 = mode B heading-hold
+constexpr uint8_t FLAG_RSTICK_XLATE = 1 << 4;   // 0 = right stick = rotation, 1 = translation
+constexpr uint8_t FLAG_GAIT_WAVE    = 1 << 5;   // 0 = tripod requested, 1 = wave requested
+constexpr uint8_t FLAG_REMOTE_GYRO  = 1 << 6;   // remote tilt feedforward enabled
+constexpr uint8_t FLAG_ROBOT_GYRO   = 1 << 7;   // robot stabilization feedback enabled
+
+class Comms {
+public:
+  bool begin();
+  void update();                       // call every loop; polls the radio if present
+  bool linkOk() const { return link_ok_; }
+  const CmdPacket& latest() const { return packet_; }
+
+  // Lets the serial test harness (or, later, real radio) inject a packet directly.
+  void injectPacket(const CmdPacket& p) { packet_ = p; last_rx_ms_ = millis(); }
+
+private:
+  CmdPacket packet_;
+  uint32_t last_rx_ms_ = 0;
+  bool link_ok_ = false;
+  bool radio_present_ = false;
+};