feat(crdt): add @robonen/crdt package

Hand-built CRDT primitives: Lamport clock + version vectors, op-log,
LWW register/map, RGA sequence, fractional indexing, marks store, sync
encode, and doc/replica. Includes eslint flat config + composite tsconfig.

core/crdt/eslint.config.ts

@@ -0,0 +1,3 @@
+import { base, compose, imports, stylistic, typescript } from '@robonen/eslint';
+
+export default compose(base, typescript, imports, stylistic);