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/.github/actions/fmt/action.yml

@@ -0,0 +1,16 @@
+name: fmt
+description: Format Rust code via rustfmt
+runs:
+  using: "composite"
+  steps:
+    - name: Install Rust
+      uses: dtolnay/rust-toolchain@1.84.0
+      with:
+        components: rustfmt
+
+    - name: Rust cache
+      uses: swatinem/rust-cache@v2
+
+    - name: cargo fmt
+      run: cargo fmt --all -- --check
+      shell: sh