Extract engine from citation-evidence umbrella (CENG-WP-0001)

Bootstrap @citation-evidence/engine as a standalone TypeScript package
with shared types and engine services copied from the umbrella MVP.
All 89 tests pass with lint and typecheck clean.

src/engine/repos/index.ts

@@ -0,0 +1,12 @@
+export {
+  createInMemoryRepos,
+  type InMemoryRepos,
+  type DocumentRepository,
+  type RepresentationRepository,
+  type AnnotationRepository,
+  type EvidenceItemRepository,
+} from "./in-memory";
+export {
+  createInMemorySessionRepository,
+  type SessionRepository,
+} from "./in-memory-sessions";