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/doc/helgobox/modules/ROOT/pages/key-concepts.adoc

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+= Key concepts
+
+[[instance]]
+== Instance
+
+Helgobox is an instrument plug-in.
+That means you can add multiple instances of it, just as you would add multiple instances of a synth or effect.
+For example, you could place one instance on the monitoring FX chain and two instances somewhere in your project.
+
+[[instance-id]]
+== Instance ID
+
+A *randomly assigned*, *non-persistent* identifier that identifies a particular <<instance>> and is *guaranteed to be unique* within one REAPER session.
+
+CAUTION: This ID can change after a restart of REAPER! If you are looking for something that doesn't change, use the <<instance-key>>.
+
+[[instance-key]]
+== Instance key
+
+A *user-defined*, *persistent* identifier that identifies a particular <<instance>>.
+The instance key gets relevant if you want to connect to an instance from an xref:app.adoc[] running in xref:app.adoc#app-remote-mode[].
+
+You can change the instance key of an instance by changing the xref:realearn::further-concepts/unit.adoc#unit-key[] of the instance's xref:realearn::key-concepts.adoc#unit[main unit] -- they are the same.
+
+
+.Make your instance keys unique!
+CAUTION: Because the instance key is user-defined, uniqueness can't be guaranteed. For example, if you just copy and paste a Helgobox FX instance, you end up with two instances that have the same key. You should avoid this.