Honor memory_mode over legacy cpu_offload and prefetch dense-quant transformer shards

plan_diffusion_memory only applies the legacy cpu_offload override when no
memory_mode was supplied, matching the documented API contract that
memory_mode overrides cpu_offload when set; an explicit fast request now
stays resident even if the old flag is also enabled.

The transformer-quant dense path fetches the base repo's transformer/
shards inside the locked finalize phase, where unload and cancellation
cannot preempt the multi-GB download. The load worker now widens the
preemptible prefetch to include those shards when that path can actually
run: quant requested and supported for the device, scheme resolvable, and
no pre-quantized checkpoint shortcutting the dense build.

studio/backend/tests/test_diffusion_memory.py

@@ -270,6 +270,21 @@ def test_explicit_cpu_offload_overrides_resident_auto_choice():
     assert any("explicit cpu_offload" in r for r in plan.reasons)
 
 
+def test_explicit_memory_mode_wins_over_legacy_cpu_offload():
+    # The API documents memory_mode as overriding cpu_offload when set: fast +
+    # the legacy flag must stay resident, not silently downgrade to offload.
+    plan = plan_diffusion_memory(
+        target = _target(),
+        device_memory = _discrete(80000),
+        model_dense_mib = 4000,
+        runtime_headroom_mib = 2000,
+        requested_mode = MEMORY_MODE_FAST,
+        explicit_offload = True,
+    )
+    assert plan.offload_policy == OFFLOAD_NONE
+    assert not any("explicit cpu_offload" in r for r in plan.reasons)
+
+
 def test_explicit_cpu_offload_ignored_on_cpu_target():
     plan = plan_diffusion_memory(
         target = _target(device = "cpu", backend = "cpu", supports_offload = False),