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/PacketBuilder.h

@@ -0,0 +1,21 @@
+#pragma once
+#include <HexapodProtocol.h>
+#include "Sticks.h"
+#include "RemoteIMU.h"
+#include "RemoteState.h"
+
+// The inverse of the robot's InputMapper: turns physical stick axes, handset
+// tilt and operator state into a CmdPacket. It only fills fields per the
+// protocol contract (HexapodProtocol.h); all physical scaling and mode
+// interpretation stays on the robot.
+class PacketBuilder {
+public:
+  CmdPacket build(const StickFrame& s, const RemoteIMU& imu, const RemoteState& st, uint32_t now_ms);
+
+private:
+  uint8_t seq_ = 0;
+  bool walk_seen_ = false;
+  uint32_t last_walk_input_ms_ = 0;
+
+  static int16_t tiltToWire(float deg, float blend);
+};