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/tsconfig.src.json

@@ -0,0 +1,9 @@
+{
+  "extends": "@robonen/tsconfig/tsconfig.base.json",
+  "compilerOptions": {
+    "composite": true,
+    "types": ["node"],
+    "tsBuildInfoFile": "./node_modules/.tmp/tsconfig.src.tsbuildinfo"
+  },
+  "include": ["src/**/*.ts"]
+}