feat(FN-4688): complete Step 2 — use range stats in landed commit lookup

Fusion-Task-Id: FN-4688
Fusion-Task-Lineage: 49abde1d-3480-4127-beb0-5dadce30b9e1

packages/engine/src/__tests__/self-healing.test.ts

@@ -2380,6 +2380,65 @@ describe("SelfHealingManager", () => {
       managerWithRecovery.stop();
     });
 
+    it("uses rebase range shortstat and propagates rebaseBaseSha when mergeDetails provides it", async () => {
+      const managerWithRecovery = new SelfHealingManager(store, {
+        rootDir: "/tmp/test-project",
+      });
+      (store.getSettings as ReturnType<typeof vi.fn>).mockResolvedValue({
+        taskStuckTimeoutMs: 60_000,
+      });
+      const staleUpdatedAt = new Date(Date.now() - 61_000).toISOString();
+
+      (store.listTasks as ReturnType<typeof vi.fn>).mockResolvedValue([
+        {
+          id: "FN-2901",
+          column: "in-review",
+          status: "merging",
+          error: null,
+          paused: false,
+          worktree: "/tmp/test-project/.worktrees/fn-2901",
+          branch: "fusion/fn-2901",
+          baseCommitSha: "base901",
+          updatedAt: staleUpdatedAt,
+          steps: [{ name: "Ship it", status: "done" }],
+          workflowStepResults: [],
+          mergeDetails: { rebaseBaseSha: "rebasebase901" },
+          log: [],
+        },
+      ]);
+      mockedExistsSync.mockReturnValue(true);
+      mockedExecSync.mockImplementation((command) => {
+        const cmd = String(command);
+        if (cmd.includes("git log") && cmd.includes("Fusion-Task-Id: FN-2901")) {
+          return "rangeSha901\u001ffeat: ship something opaque\n" as any;
+        }
+        if (cmd.includes("git diff --shortstat") && cmd.includes("rebasebase901..rangeSha901")) {
+          return " 4 files changed, 104 insertions(+), 1 deletion(-)\n" as any;
+        }
+        return "" as any;
+      });
+
+      const result = await managerWithRecovery.recoverInterruptedMergingTasks();
+
+      expect(result).toBe(1);
+      expect(mockedExecSync.mock.calls.some(([cmd]) => String(cmd).includes("git diff --shortstat") && String(cmd).includes("rebasebase901..rangeSha901"))).toBe(true);
+      expect(mockedExecSync.mock.calls.some(([cmd]) => String(cmd).includes("git show --shortstat") && String(cmd).includes("rangeSha901"))).toBe(false);
+      expect(store.updateTask).toHaveBeenCalledWith("FN-2901", {
+        status: null,
+        error: null,
+        mergeRetries: 0,
+        mergeDetails: expect.objectContaining({
+          commitSha: "rangeSha901",
+          rebaseBaseSha: "rebasebase901",
+          filesChanged: 4,
+          insertions: 104,
+          deletions: 1,
+        }),
+      });
+
+      managerWithRecovery.stop();
+    });
+
     it("finalizes stale merging tasks when baseCommitSha was advanced past the landed commit", async () => {
       // Reproduces the case where the merger fast-forward-rebased the task branch
       // and updated baseCommitSha to the new HEAD; the bounded `base..HEAD` range

packages/engine/src/self-healing.ts

@@ -731,6 +731,7 @@ export class SelfHealingManager {
     // miss; (3) catches commits authored before the trailer was introduced.
 
     // ── (1) Stored sha ────────────────────────────────────────────────────
+    const rebaseBaseSha = task.mergeDetails?.rebaseBaseSha;
     const storedSha = task.mergeDetails?.commitSha;
     if (storedSha) {
       try {
@@ -744,13 +745,12 @@ export class SelfHealingManager {
         );
         const [sha, subject = "", body = ""] = stdout.trim().split("\x1f");
         if (sha && commitOwnedByTask(task.id, task.lineageId, subject, body)) {
-          const commit: LandedTaskCommit = { sha, subject };
+          const commit: LandedTaskCommit = { sha, subject, rebaseBaseSha };
           try {
-            const stats = await execAsync(`git show --shortstat --format= ${shellQuote(sha)}`, {
-              cwd: this.options.rootDir,
-              maxBuffer: 1024 * 1024,
-            });
-            Object.assign(commit, parseShortstat(stats.stdout));
+            const shortstat = await this.readShortstatForSha(sha, rebaseBaseSha);
+            if (shortstat) {
+              Object.assign(commit, shortstat);
+            }
           } catch { /* stats are optional */ }
           return commit;
         }
@@ -829,13 +829,12 @@ export class SelfHealingManager {
     const [sha, subject] = firstLine.split("\x1f");
     if (!sha) return null;
 
-    const commit: LandedTaskCommit = { sha, subject };
+    const commit: LandedTaskCommit = { sha, subject, rebaseBaseSha };
     try {
-      const stats = await execAsync(`git show --shortstat --format= ${shellQuote(sha)}`, {
-        cwd: this.options.rootDir,
-        maxBuffer: 1024 * 1024,
-      });
-      Object.assign(commit, parseShortstat(stats.stdout));
+      const shortstat = await this.readShortstatForSha(sha, rebaseBaseSha);
+      if (shortstat) {
+        Object.assign(commit, shortstat);
+      }
     } catch (err: unknown) {
       const errorMessage = err instanceof Error ? err.message : String(err);
       log.warn(