feat(model): add Proposal.original for F12 optimistic-apply revert (#64)

test/model.test.ts

@@ -1,4 +1,5 @@
-import { describe, it, expect } from "vitest";
+import assert from "node:assert";
+import { describe, it, test, expect } from "vitest";
 import {
   SCHEMA_VERSION,
   emptyArtifact,
@@ -179,6 +180,19 @@ describe("unknown-field preservation (F5 SLICE-2, INV-15)", () => {
   });
 });
 
+test("serializeArtifact round-trips Proposal.original", () => {
+  const a = emptyArtifact("doc.md");
+  a.anchors["a_1"] = { fingerprint: { text: "new", before: "", after: "", lineHint: 0 } };
+  a.proposals.push({
+    id: "pr_1", anchorId: "a_1", replacement: "new",
+    author: { kind: "agent", id: "claude", agent: { sdk: "@cline/sdk", model: "sonnet", sessionId: "s" } },
+    createdAt: "2026-06-26T00:00:00.000Z", original: "old", granularity: "block",
+  });
+  const json = serializeArtifact(a);
+  assert.match(json, /"original": "old"/);
+  assert.match(json, /"granularity": "block"/);
+});
+
 describe("attributions[] (F3 SLICE-1)", () => {
   it("round-trips an attribution record through serialize → parse", () => {
     const a = emptyArtifact("docs/x.md");