feat(chord): add JSON delta tracking

packages/chord/PLANNING.md

@@ -404,7 +404,7 @@ Explicit non-goals:
 - automatic unchanged-value suppression; and
 - high-frequency stream transport.
 
-A later delta primitive requires a concrete feature and a separate snapshot/delta/gap-recovery design.
+Chord now exposes a standalone intent-preserving JSON delta primitive for durable output and other explicit consumers. Replicated service state still uses complete snapshots and ordered latest-value updates; adopting deltas there requires a separate snapshot/delta/gap-recovery design.
 
 ## 9. Symmetric RPC plumbing
 

packages/chord/README.md

@@ -36,6 +36,10 @@ The design has a few connected pieces:
   ordered updates, and become unready on disconnect or replacement until they
   are rehydrated.
 
+- **Delta tracking** records mutations to plain JSON as compact operations. It
+  preserves append, front-truncation, and array-splice intent, supports durable
+  base batches, and validates untrusted operations before applying them.
+
 - **Remote service sources** advertise services available outside a facet host
   and open bindings for the services its facets require. Bindings carry logical
   calls and subscriptions through an application-supplied adapter. Chord
@@ -57,6 +61,26 @@ the root API.
 Chord-owned identifiers use the `chord.*` namespace and its reserved service
 prefix is `$chord.*`.
 
+## Tracking JSON deltas
+
+Import the standalone delta primitive from `@earendil-works/chord/delta`:
+
+```ts
+import { apply, track } from "@earendil-works/chord/delta";
+
+const changes = track({ output: "", count: 0 });
+changes.flush(); // opening base batch
+changes.state.output += "done\n";
+changes.state.count += 1;
+
+const ops = changes.flush();
+const replica = apply({ output: "", count: 0 }, ops);
+```
+
+The first flush is always a complete base batch. Later flushes contain path-based
+changes. String assignments preserve pure appends and rolling-window movement as
+append and front-truncate operations; unrelated rewrites fall back to a set.
+
 ## Bundling and loading facets
 
 `@earendil-works/chord/bundler` uses esbuild to turn ESM or TypeScript application

packages/chord/package.json

@@ -16,6 +16,11 @@
 			"types": "./dist/context/index.d.ts",
 			"import": "./dist/context/index.js"
 		},
+		"./delta": {
+			"source": "./src/delta/index.ts",
+			"types": "./dist/delta/index.d.ts",
+			"import": "./dist/delta/index.js"
+		},
 		"./bundler": {
 			"source": "./src/bundler.ts",
 			"types": "./dist/bundler.d.ts",