Vendor helgoboss/helgobox (ReaLearn) as basis for custom UI fork

Stripped upstream git history; starting point for replacing the native
SWELL/Win32 mapping UI with something more suited to bulk editing.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

plugin-reaper-realearn/main/lib/helgoboss-learn/README.md

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+# helgoboss-learn
+
+Rust crate that provides DAW-agnostic MIDI-learn functionality.
+
+## Status
+
+At the moment, development of this crate is still closely coupled to [ReaLearn](https://github.com/helgoboss/realearn),
+currently the only downstream crate. Therefore you won't find it on [crates.io](https://crates.io/) or
+[docs.rs](https://docs.rs/).
+
+However, it's designed to be completely independent of ReaLearn and REAPER. So if in future another potential
+downstream crate pops up, it will be easy to make *helgoboss-learn* and independent library.