fix(context-engine): lazy-load legacy engine registration

src/context-engine/legacy.registration.ts

@@ -1,10 +1,28 @@
 import { registerContextEngineForOwner } from "./registry.js";
+import type { ContextEngine } from "./types.js";
+
+const LEGACY_CONTEXT_ENGINE_CANDIDATES = ["./legacy.js", "./legacy.ts"] as const;
+
+type LegacyContextEngineModule = {
+  LegacyContextEngine: new () => ContextEngine;
+};
+
+async function loadLegacyContextEngineModule(): Promise<LegacyContextEngineModule> {
+  for (const candidate of LEGACY_CONTEXT_ENGINE_CANDIDATES) {
+    try {
+      return (await import(candidate)) as LegacyContextEngineModule;
+    } catch {
+      // Try runtime/source candidates in order.
+    }
+  }
+  throw new Error("Failed to load legacy context engine runtime.");
+}
 
 export function registerLegacyContextEngine(): void {
   registerContextEngineForOwner(
     "legacy",
     async () => {
-      const { LegacyContextEngine } = await import("./legacy.js");
+      const { LegacyContextEngine } = await loadLegacyContextEngineModule();
       return new LegacyContextEngine();
     },
     "core",