Phase 8: add compaction runtime state — pendingCompactionSource container for §8.

Transient CompactionRuntime holds pending source between input, before_compact, and session_compact hooks.

Co-authored-by: Cursor <cursoragent@cursor.com>

src/compaction.ts

@@ -0,0 +1,12 @@
+import type { CompactionSource } from "./state.js";
+
+/** Runtime compaction-source detection between input → before_compact → session_compact (SPEC §8). */
+export interface CompactionRuntime {
+	pendingCompactionSource: CompactionSource | null;
+}
+
+export function createCompactionRuntime(): CompactionRuntime {
+	return {
+		pendingCompactionSource: null,
+	};
+}