Draft: shared protocol, remote firmware, project docs

Not yet verified on hardware.

- shared/HexapodProtocol: CmdPacket (now 20 bytes, adds remote_roll /
  remote_pitch), FLAG_* bits and radio settings as the single protocol
  definition; static payload size = sizeof(CmdPacket) on both radios.
- robot: use the shared protocol; vx = forward (harness w/x <-> q/e);
  InputMapper adds remote tilt on top of the right stick when
  FLAG_REMOTE_GYRO is set, and robot.ino no longer zeroes stick
  roll/pitch when it's off.
- remote (Teensy 4.1): Sticks (calibration, EEPROM), RemoteIMU,
  RemoteState (interlocks, link-loss disarm, smoothing), PacketBuilder,
  RadioLink, NextionUI, SerialConsole.
- CLAUDE.md, README.md, remote/README.md, remote/NEXTION.md.

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

2026-current/remote/RadioLink.h

@@ -0,0 +1,22 @@
+#pragma once
+#include <HexapodProtocol.h>
+
+// nRF24L01 transmitter. Uses hardware auto-ack to measure the link: the
+// robot's radio acks every packet it receives, so an ack = the robot heard us.
+class RadioLink {
+public:
+  bool begin();                         // false if the radio chip didn't respond
+  bool send(const CmdPacket& p);        // blocking, returns true if acked
+  void update(uint32_t now_ms);         // refreshes linkOk()
+
+  bool present() const { return present_; }
+  bool linkOk() const  { return link_ok_; }
+  uint8_t qualityPct() const;           // acked share of the last 32 packets
+
+private:
+  bool present_ = false;
+  bool link_ok_ = false;
+  uint32_t last_ack_ms_ = 0;
+  bool ever_acked_ = false;
+  uint32_t history_ = 0;                // 1 bit per packet, newest in bit 0
+};