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/oplog/index.ts

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

core/crdt/src/oplog/op-log.ts

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+import type { OpId } from '../clock';
+import { VersionVector } from '../clock';
+
+/** Anything carrying an op id can live in the log. */
+export interface HasOpId {
+  readonly id: OpId;
+}
+
+/**
+ * An append-only log of operations with a version vector for deduplication and
+ * delta computation. The op shape is domain-specific; the log only reads `id`.
+ */
+export class OpLog<Op extends HasOpId> {
+  private readonly ops: Op[] = [];
+  private readonly vv = new VersionVector();
+
+  /** Append an op unless already seen. Returns `true` if appended. */
+  append(op: Op): boolean {
+    if (this.vv.has(op.id))
+      return false;
+
+    this.ops.push(op);
+    this.vv.observe(op.id);
+    return true;
+  }
+
+  has(id: OpId): boolean {
+    return this.vv.has(id);
+  }
+
+  get version(): VersionVector {
+    return this.vv;
+  }
+
+  all(): readonly Op[] {
+    return this.ops;
+  }
+
+  /** Ops a remote replica (described by its version vector) hasn't seen. */
+  delta(remote: VersionVector): Op[] {
+    return this.ops.filter(op => !remote.has(op.id));
+  }
+}