Robot: Teensy 3.6 with the old robot's servo wiring and calibration

- LEGACY/Hexapod_Teensy/: old robot sketch (reference only).
- platformio.ini: robot env targets teensy36 (owner-confirmed board).
- Config.h: per-servo SERVOS table (pin, pulse at 0 deg, signed
  us/deg, pulse limits) built from the old sketch's pins, ranges, trims
  and the old Unity servo directions. Rear-left/front-left coxa offsets
  set to 0: the owner re-horns those two servos (the old +-90 left them
  at the end of travel; the sim showed them clamping).
- ServoDriver: 18 servos on hardware PWM (50 Hz, 16-bit); the Teensy 3.6
  Servo library stops at 12.
- Radio on CE 39 / CSN 15 with SPI SCK on pin 14.
- sim: analogWrite/PWM shims, pulses decoded through SERVOS (API v6).
- Docs: CLAUDE.md, READMEs.

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

2026-current/sim/shim/SPI.h

@@ -1,2 +1,12 @@
 #pragma once
-// Nothing needed: the simulated RF24 doesn't talk SPI.
+#include <Arduino.h>
+
+// The simulated RF24 doesn't talk SPI; this only has to accept the pin setup.
+class SPIClass {
+public:
+  void begin() {}
+  void setSCK(uint8_t) {}
+  void setMOSI(uint8_t) {}
+  void setMISO(uint8_t) {}
+};
+extern SPIClass SPI;