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/StateMachine.cpp

@@ -0,0 +1,22 @@
+#include "StateMachine.h"
+#include <Arduino.h>
+
+void StateMachine::update(bool armed, bool stand_request, bool walk_active, float dt) {
+  if (!armed) {
+    state_ = RobotState::DISARMED;
+    blend_ = 0;
+    return;
+  }
+  if (state_ == RobotState::DISARMED) {
+    state_ = RobotState::SIT;   // arming always lands in sit first, never straight to stand
+  }
+
+  float target_blend = stand_request ? 1.0f : 0.0f;
+  float step = BLEND_RATE * dt;
+  if (blend_ < target_blend)      blend_ = min(blend_ + step, target_blend);
+  else if (blend_ > target_blend) blend_ = max(blend_ - step, target_blend);
+
+  if (blend_ <= 0.001f)      state_ = RobotState::SIT;
+  else if (blend_ >= 0.999f) state_ = walk_active ? RobotState::WALKING : RobotState::STAND;
+  else                        state_ = RobotState::STAND;   // mid-transition
+}