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/clock/version-vector.ts

@@ -0,0 +1,41 @@
+import type { OpId, SiteId } from './id';
+
+/**
+ * Tracks the highest clock seen per site, assuming each site emits dense clocks
+ * (1, 2, 3, …). Used to deduplicate ops and to compute deltas during sync.
+ */
+export class VersionVector {
+  private readonly clocks = new Map<SiteId, number>();
+
+  /** Record that an op has been seen. */
+  observe(id: OpId): void {
+    if (id.clock > this.get(id.site))
+      this.clocks.set(id.site, id.clock);
+  }
+
+  /** Highest clock seen for a site (0 if none). */
+  get(site: SiteId): number {
+    return this.clocks.get(site) ?? 0;
+  }
+
+  /** Whether an op id has already been seen. */
+  has(id: OpId): boolean {
+    return this.get(id.site) >= id.clock;
+  }
+
+  /** Plain-object snapshot for transport. */
+  toJSON(): Record<SiteId, number> {
+    return Object.fromEntries(this.clocks);
+  }
+
+  static fromJSON(snapshot: Record<SiteId, number>): VersionVector {
+    const vv = new VersionVector();
+    for (const site in snapshot)
+      vv.clocks.set(site, snapshot[site]!);
+    return vv;
+  }
+
+  clone(): VersionVector {
+    return VersionVector.fromJSON(this.toJSON());
+  }
+}