fix(terminal): a remote shell load keeps the remote scrollback (#601/#602 integration)

A remote session's local pane hosts the remote tmux client, so it is always
in the alternate screen. #601's alt=recording fallback therefore threw away
the scrollback #602 had just spliced in from the remote tmux on every remote
shell load. The fallback now applies only when no remote scrollback was
read (the read failed, or the session is not remote).

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

test/routes/session-routes.test.ts

@@ -1349,6 +1349,28 @@ describe('session-routes', () => {
         }
       });
 
+      it("keeps a remote session's spliced remote scrollback, and falls back only when that read failed", async () => {
+        // The remote session's local pane hosts the remote tmux client, so it is
+        // always in the alternate screen; the history comes from the remote tmux.
+        const spy = install(true);
+        const session = harness.ctx._session as unknown as { remote?: unknown };
+        session.remote = { hostId: 'h1', label: 'proxx', host: 'proxx.lan', username: 'tim', remotePath: '/home/tim' };
+        try {
+          remoteScrollback.mockResolvedValueOnce('remote line 1\n');
+          const spliced = await load(`full=1&tail=${1024 * 1024}&alt=recording&lines=10000`);
+          expect(spy).toHaveBeenCalledTimes(1);
+          expect(spliced.source).toBe('mux-full-history');
+
+          spy.mockClear();
+          remoteScrollback.mockResolvedValueOnce(null);
+          const fallback = await load(`full=1&tail=${1024 * 1024}&alt=recording&lines=10000`);
+          expect(spy).toHaveBeenCalledTimes(2);
+          expect(fallback.source).toBe('mux-visible');
+        } finally {
+          session.remote = undefined;
+        }
+      });
+
       it('keeps the full capture without the opt-in (agents, Load full history, scroll pulls)', async () => {
         const spy = install(true);
         const data = await load(`full=1&tail=${1024 * 1024}&lines=10000`);