Phase 16 review fixes: engine-switch unload, sd.cpp error mapping, per-image seeds, Qwen sampler

Address review feedback on #6724:
- engine router: unload the engine being deactivated on a switch, so the old
  model is not left resident-but-unreachable (the evictor only targets the active
  engine).
- generate route: sd.cpp execution errors (nonzero exit / timeout / missing
  output) now map to 500, not 409 (which only means not-loaded / cancelled).
- native batch: return per-image seeds and persist the actual seed for each image
  so every batch image is reproducible.
- Qwen-Image native path: apply --sampling-method euler --flow-shift 3 per the
  stable-diffusion.cpp docs; other families keep sd-cli defaults.
- honor speed_mode (native --diffusion-fa) and, off-CPU, memory_mode/cpu_offload
  offload flags on the native load instead of hardcoding them off.
- fail the load when the sd-cli binary is present but not runnable (version()
  now returns None on exec error / nonzero exit).
- size estimate: only treat the transformer asset as a possible local path.

studio/backend/tests/test_sd_cpp_engine.py

@@ -95,7 +95,9 @@ def test_engine_version_parsed_and_cached(tmp_path, monkeypatch):
 
     def _fake_run(*_a, **_k):
         calls["n"] += 1
-        return types.SimpleNamespace(stdout = "stable-diffusion.cpp version master-721\n", stderr = "")
+        return types.SimpleNamespace(
+            stdout = "stable-diffusion.cpp version master-721\n", stderr = "", returncode = 0
+        )
 
     monkeypatch.setattr(eng.subprocess, "run", _fake_run)
     assert e.version() == "stable-diffusion.cpp version master-721"