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/lamport.ts

@@ -0,0 +1,29 @@
+import type { OpId, SiteId } from './id';
+
+/**
+ * A Lamport clock for one site: hands out monotonically increasing op ids and
+ * advances past observed remote ops so locally-generated ids stay causally later.
+ */
+export class LamportClock {
+  private counter: number;
+
+  constructor(public readonly site: SiteId, start = 0) {
+    this.counter = start;
+  }
+
+  /** Generate the next op id for a local operation. */
+  tick(): OpId {
+    this.counter += 1;
+    return { site: this.site, clock: this.counter };
+  }
+
+  /** Advance past a remote op so future local ticks are causally after it. */
+  observe(id: OpId): void {
+    if (id.clock > this.counter)
+      this.counter = id.clock;
+  }
+
+  get value(): number {
+    return this.counter;
+  }
+}