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/vitest.config.ts

@@ -0,0 +1,7 @@
+import { defineConfig } from 'vitest/config';
+
+export default defineConfig({
+  test: {
+    environment: 'node',
+  },
+});