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/NextionUI.h

@@ -8,8 +8,9 @@
 //      '#' <len> <cmd> [value]        e.g. printh 23 01 41   (ToggleArm)
 //                                          printh 23 02 56 + prints hSpeed.val,1
 //    Command bytes are listed in NEXTION.md and in NextionUI.cpp.
-//    Native Nextion return frames (terminated FF FF FF) are also parsed so a
-//    display reset (0x88 "ready") triggers a full redraw.
+//    Native Nextion return frames (terminated FF FF FF) are also parsed: a
+//    display reset (0x88 "ready") triggers a full redraw, and the old HMI's
+//    upload button sends a standard touch event (0x65) that's recognized.
 //
 //  remote -> Nextion: plain instructions ("bArm.txt=\"ARMED\"" + FF FF FF).
 //
@@ -23,12 +24,16 @@ public:
   bool poll(UiEvent& out);                 // call every loop; returns one decoded event at a time
   void render(const UiView& v, uint32_t now_ms);
 
+  // Nextion Editor upload through the remote's USB port. Never returns.
+  [[noreturn]] void runUploadPassthrough();
+
 private:
   enum class Rx : uint8_t { Idle, CustomLen, CustomBody, Native };
   Rx rx_ = Rx::Idle;
   uint8_t body_[4];
   uint8_t body_len_ = 0, body_got_ = 0;
-  uint8_t native_first_ = 0, native_len_ = 0, ff_run_ = 0;
+  uint8_t native_buf_[8];
+  uint8_t native_len_ = 0, ff_run_ = 0;
 
   UiView shown_;
   bool force_full_ = true;
@@ -39,4 +44,5 @@ private:
   void pushState(const UiView& v, bool full);
   void pushLink(const UiView& v);
   bool decodeCustom(UiEvent& out);
+  bool decodeNative(UiEvent& out);
 };