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/src/sync/__test__/encode.test.ts

@@ -0,0 +1,20 @@
+import { describe, expect, it } from 'vitest';
+import { VersionVector, opId } from '../../clock';
+import { decodeOps, decodeStateVector, encodeOps, encodeStateVector } from '..';
+
+describe('sync encoding', () => {
+  it('round-trips a version vector through bytes', () => {
+    const vv = new VersionVector();
+    vv.observe(opId('a', 3));
+    vv.observe(opId('b', 1));
+
+    const restored = decodeStateVector(encodeStateVector(vv));
+    expect(restored.get('a')).toBe(3);
+    expect(restored.get('b')).toBe(1);
+  });
+
+  it('round-trips an op batch through bytes', () => {
+    const ops = [{ id: opId('a', 1), kind: 'insert', value: 'x' }];
+    expect(decodeOps(encodeOps(ops))).toEqual(ops);
+  });
+});

core/crdt/src/sync/encode.ts

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+import { VersionVector } from '../clock';
+
+const encoder = new TextEncoder();
+const decoder = new TextDecoder();
+
+/**
+ * Transport-agnostic wire encoding. v1 is JSON-over-bytes — simple and
+ * debuggable; a compact varint format is a later optimization with no API change.
+ */
+export function encodeJson(value: unknown): Uint8Array {
+  return encoder.encode(JSON.stringify(value));
+}
+
+export function decodeJson<T>(bytes: Uint8Array): T {
+  return JSON.parse(decoder.decode(bytes)) as T;
+}
+
+/** Encode a version vector for a "what do you have?" sync handshake. */
+export function encodeStateVector(vv: VersionVector): Uint8Array {
+  return encodeJson(vv.toJSON());
+}
+
+export function decodeStateVector(bytes: Uint8Array): VersionVector {
+  return VersionVector.fromJSON(decodeJson(bytes));
+}
+
+/** Encode a batch of ops (the delta or a full snapshot). */
+export function encodeOps<Op>(ops: readonly Op[]): Uint8Array {
+  return encodeJson(ops);
+}
+
+export function decodeOps<Op>(bytes: Uint8Array): Op[] {
+  return decodeJson<Op[]>(bytes);
+}

core/crdt/src/sync/index.ts

@@ -0,0 +1 @@
+export * from './encode';