Compile .py to .mpy with mpy-cross on upload

[bdbarnett]

Summary

Add optional host-side compilation of .py files to .mpy via mpy-cross when transferring files to a MicroPython board (CLI put/cp and File Transfer UI).

Motivation

Uploading bytecode instead of source:

• Uses less flash
• Speeds up imports (no parse/compile on device)
• Matches how mip often installs packages (.mpy by default)

mpftp already builds host mpy-cross as part of firmware builds (firmware_engine.py), but file transfer always sends .py as-is. Exposing compile-on-upload would close that gap for app/lib deployment workflows.

Proposed behavior

1. Opt-in setting / CLI flag, e.g. mpftp.compileOnUpload and --mpy / --compile on put / cp.
2. When enabled and the destination is MicroPython:
   • For each uploaded .py, run mpy-cross on the host
   • Upload the resulting .mpy (same basename, .mpy extension)
   • Skip files that should remain source (boot.py, main.py, or a configurable allowlist/denylist)
3. Compatibility: pass the correct -march / mpy version for the connected firmware (probe board or reuse firmware-workspace mpy-cross built for that tree). Mismatch should fail clearly rather than silently upload incompatible bytecode.
4. Tooling discovery (in order): firmware-workspace mpy-cross binary → PATH → error with install/build hint.
5. CircuitPython: out of scope or explicitly unsupported (different bytecode / tooling), unless a clear CP path exists later.

Acceptance criteria

• CLI can upload a .py as .mpy via mpy-cross with an explicit flag
• Setting enables the same for File Transfer uploads
• Incompatible mpy-cross / board combo produces a clear error
• boot.py / main.py (and documented exceptions) are not rewritten to .mpy by default
• Docs: user guide + CLI help covering flags, discovery, and compatibility

Notes

Related existing pieces:

• Firmware build already invokes make -C mpy-cross
• mip has --mpy / source install choices; this request is about user file transfer, not package install