refactor: isolate dispatch scheduling core

crates/aether-dispatch-core/src/effects.rs

@@ -0,0 +1,16 @@
+#[derive(Debug, Clone, PartialEq, Eq, serde::Serialize, serde::Deserialize)]
+pub enum DispatchEffectKind {
+    CandidateFailed,
+    RateLimited,
+    Succeeded,
+}
+
+#[derive(Debug, Clone, PartialEq, Eq, serde::Serialize, serde::Deserialize)]
+pub struct DispatchEffect {
+    pub kind: DispatchEffectKind,
+    pub provider_id: String,
+    pub endpoint_id: String,
+    pub key_id: Option<String>,
+    pub candidate_index: u32,
+    pub reason: Option<String>,
+}