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/UiTypes.h

@@ -23,6 +23,7 @@ enum class UiCmd : uint8_t {
   ZeroGyro,            // capture the handset's current attitude as "level"
   CalStartSave,        // start sweep calibration, or save it if running
   CalCancel,
+  NextionUpload,       // USB <-> display passthrough for Nextion Editor (handled in remote.ino)
 };
 
 struct UiEvent {