Replace per-setting settings-event macro expansion with shared generic registration

The register_settings_events! macro expanded a full registration
function for every setting. Move that body into
settings::registration, where it is generic over only the settings
group and value types, so settings that share those types share one
compiled instantiation. The macro now only builds a SettingCallbacks
struct of function pointers at its expansion site, which keeps the
method resolution of the old expanded code. Emit change events through
the new SettingChangeEvent trait so define_setting! and
implement_setting_for_enum! generate one concat_idents! block per
setting instead of four.

Co-Authored-By: Warp <agent@warp.dev>

Author: vorporeal

crates/settings/src/lib.rs

@@ -1,6 +1,7 @@
 #[macro_use]
 pub mod macros;
 pub mod manager;
+pub mod registration;
 pub mod schema;
 
 // Re-export commonly used types and traits
@@ -640,6 +641,17 @@ pub trait Setting {
     fn is_value_explicitly_set(&self) -> bool;
 }
 
+/// A trait that maps a setting to the change event of its settings group.
+///
+/// The setting macros implement this trait for each setting they define. This
+/// lets the shared `Setting` implementations construct the correct group event
+/// variant without expanding per-setting event code at each emit site.
+pub trait SettingChangeEvent: Setting {
+    /// Returns the group event that reports a change to this setting for the
+    /// given reason.
+    fn change_event(reason: ChangeEventReason) -> <Self::Group as Entity>::Event;
+}
+
 /// Shared persistence operations for typed settings backed by secure storage.
 ///
 /// Implementors remain responsible for routing their [`Setting`] lifecycle