fix(app): open Changes on the populated comparison (#3636)

* fix(app): restore smart Changes comparison selection

The tab migration persisted the selected comparison, pinning Changes to an empty view after checkout state changed. Keep comparison overrides ephemeral per surface and expire them when dirtiness changes, while preserving only presentation state in tabs.

* chore(app): date Changes compatibility cleanup

packages/app/src/stores/workspace-layout-store.test.ts

@@ -484,7 +484,7 @@ describe("workspace-layout-store actions", () => {
     ).toEqual(override);
   });
 
-  it("keeps every Changes state field independent across explicit instances", () => {
+  it("keeps every Changes presentation field independent across explicit instances", () => {
     const workspaceKey = createWorkspaceKey();
     const store = workspaceLayoutStore.getState();
     const first = store.openTab({
@@ -498,8 +498,6 @@ describe("workspace-layout-store actions", () => {
       intent: "new",
     })!;
     const changed: ChangesState = {
-      mode: "base",
-      baseRef: "origin/main",
       layout: "split" as const,
       wrapLines: true,
       hideWhitespace: true,
@@ -811,8 +809,6 @@ describe("workspace-layout-store actions", () => {
       .getState()
       .openTab({ workspaceKey: workspaceKey, target: { kind: "working_diff" }, intent: "new" })!;
     const firstState: ChangesState = {
-      mode: "base" as const,
-      baseRef: "origin/main",
       layout: "split" as const,
       wrapLines: true,
       hideWhitespace: true,
@@ -823,8 +819,6 @@ describe("workspace-layout-store actions", () => {
       commitsCollapsed: false,
     };
     const secondState: ChangesState = {
-      mode: "base" as const,
-      baseRef: "release",
       layout: "unified" as const,
       wrapLines: false,
       hideWhitespace: false,