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/src/source/feedback_script.rs

@@ -0,0 +1,41 @@
+use crate::{FeedbackValue, PropValue};
+use base::hash_util::NonCryptoHashSet;
+use std::borrow::Cow;
+use std::error::Error;
+
+// The lifetime 'a is necessary in case we want to parameterize the lifetime
+// of the additional input dynamically. An alternative would have been to
+// require the additional input type to be static and take it by reference.
+// But that would be less generic.
+pub trait FeedbackScript<'a> {
+    type AdditionalInput: Default;
+
+    fn feedback(
+        &self,
+        input: FeedbackScriptInput,
+        additional_input: Self::AdditionalInput,
+    ) -> Result<FeedbackScriptOutput, Cow<'static, str>>;
+
+    fn used_props(&self) -> Result<NonCryptoHashSet<String>, Box<dyn Error>>;
+}
+
+pub trait PropProvider {
+    fn get_prop_value(&self, key: &str) -> Option<PropValue>;
+}
+
+impl<F> PropProvider for F
+where
+    F: Fn(&str) -> Option<PropValue>,
+{
+    fn get_prop_value(&self, key: &str) -> Option<PropValue> {
+        (self)(key)
+    }
+}
+
+pub struct FeedbackScriptInput<'a> {
+    pub prop_provider: &'a dyn PropProvider,
+}
+
+pub struct FeedbackScriptOutput {
+    pub feedback_value: FeedbackValue<'static>,
+}