Remote: match the old remote's hardware; old remote code in LEGACY

- LEGACY/remote/teensy/: old remote sketch from
  https://mangavalk.com/Hexapod/remote.git @ 492dece (reference only).
- remote/Config.h: sticks on A0-A5, Nextion on Serial1 (0/1), as in the
  old sketch; MPU6050 moved to Wire2 (25/24) since A4/A5 are Wire's pins.
- NextionUI: controls on a vscope-global "control" page added to the
  old HMI; keeps the old t1 status texts; the old page-1 b2 button (and
  serial 'U') enter Nextion upload passthrough, following the PC's baud.
- Docs: NEXTION.md, remote README wiring, CLAUDE.md.

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

2026-current/remote/remote.ino

@@ -47,12 +47,27 @@ static void dumpPacket(const CmdPacket& p) {
                 p.remote_roll, p.remote_pitch, p.body_height, p.speed_scale, p.flags);
 }
 
+// Nextion Editor upload through this USB port (the old remote's b2 button).
+// Never returns: the TX loop stops, so the robot loses the link and
+// disarms itself after LINK_TIMEOUT_SEC. Power-cycle the remote to leave.
+static void enterNextionUpload() {
+  state.handle(UiEvent{ UiCmd::Disarm, 0 }, sticks, imu, millis());
+  Serial.println("Nextion upload mode: connect Nextion Editor to this USB port and upload.");
+  Serial.println("Power-cycle the remote to leave.");
+  Serial.flush();
+  nextion.runUploadPassthrough();
+}
+
 static void pollInputs(uint32_t now_ms) {
   UiEvent ev;
-  while (nextion.poll(ev)) state.handle(ev, sticks, imu, now_ms);
+  while (nextion.poll(ev)) {
+    if (ev.cmd == UiCmd::NextionUpload) enterNextionUpload();
+    state.handle(ev, sticks, imu, now_ms);
+  }
 
   SerialConsole::Local local;
   while (console.poll(ev, local)) {
+    if (ev.cmd == UiCmd::NextionUpload) enterNextionUpload();
     if (ev.cmd != UiCmd::None) {
       bool ok = state.handle(ev, sticks, imu, now_ms);
       if (!ok && state.message(now_ms)[0]) Serial.println(state.message(now_ms));