feat(FN-4461): add transition evidence backfill script and tests

Adds a backfill script and its tests for FN-4441 transition evidence, covering two new files totaling 204 lines.

Fusion-Task-Id: FN-4461

scripts/__tests__/backfill-fn-4441-transition-evidence.test.mjs

@@ -0,0 +1,76 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+import { tsImport } from "tsx/esm/api";
+import { composeTransitionEvidence } from "../backfill-fn-4441-transition-evidence.mjs";
+
+async function loadTaskStore() {
+  const moduleUrl = new globalThis.URL("../../packages/core/src/store.ts", import.meta.url).href;
+  const mod = await tsImport(moduleUrl, import.meta.url);
+  return mod.TaskStore;
+}
+
+test("composeTransitionEvidence includes required evidence fields", () => {
+  const content = composeTransitionEvidence({
+    mergeRetries: undefined,
+    branchTip: {
+      short: "f57f70165",
+      full: "f57f70165ac154f1ca79e68510990bc61916660e",
+    },
+    mainSHAs: [
+      {
+        sha: "00c8739d5",
+        title: "feat(FN-4441): complete Step 1 — add node_modules cache layer",
+        ancestorStatus: 0,
+        ancestorOutput: "ancestor",
+      },
+      {
+        sha: "41070475b",
+        title: "feat(FN-4441): complete Step 2 — verify cache-hit integrity",
+        ancestorStatus: 0,
+        ancestorOutput: "ancestor",
+      },
+    ],
+    lastMergeError: "The previous cherry-pick is now empty",
+    resolutionTaskId: "FN-4450",
+    postState: {
+      column: "done",
+      branchDisposition: "deleted",
+    },
+  });
+
+  assert.match(content, /f57f70165/);
+  assert.match(content, /00c8739d5/);
+  assert.match(content, /41070475b/);
+  assert.match(content, /FN-4450/);
+  assert.match(content, /mergeRetries:/i);
+  assert.match(content, /last merge error:/i);
+});
+
+test("TaskStore upsertTaskDocument increments revision and round-trips latest content", async () => {
+  const TaskStore = await loadTaskStore();
+  const store = new TaskStore(process.cwd(), undefined, { inMemoryDb: true });
+
+  try {
+    await store.createTaskWithReservedId({ description: "seed" }, { taskId: "FN-4441" });
+
+    const first = await store.upsertTaskDocument("FN-4441", {
+      key: "transition-evidence",
+      content: "first",
+      author: "test",
+    });
+    const second = await store.upsertTaskDocument("FN-4441", {
+      key: "transition-evidence",
+      content: "second",
+      author: "test",
+    });
+
+    assert.equal(first.revision, 1);
+    assert.equal(second.revision, 2);
+
+    const doc = await store.getTaskDocument("FN-4441", "transition-evidence");
+    assert.equal(doc?.revision, 2);
+    assert.equal(doc?.content, "second");
+  } finally {
+    await store.close();
+  }
+});