[Refactor][Diffusion] Complete generic backend plan consumption (#7313)

Signed-off-by: specture724 <specture724@gmail.com>
Co-authored-by: Claude Opus 5 <noreply@anthropic.com>

docs/design/feature/offloader/README.md

@@ -41,7 +41,8 @@ Every backend implements `OffloadBackend`:
 - `is_enabled()` reports lifecycle state.
 
 `disable()` does not promise to restore the pipeline's original device
-placement. The caller owns any subsequent rematerialization.
+placement. It must restore usable parameter and buffer storage before releasing
+backend-owned host weights. The caller owns subsequent device placement.
 
 An enable failure must remove every partially installed hook. Rank-local
 backends restore ordinary tensors so the pipeline can be retried. A failed
@@ -54,9 +55,19 @@ must use distinct hook names and remove only hooks they own.
 ## Discovery and topology
 
 `resolve_offload_plan(pipeline, config)` is the single topology entry point.
-Every backend calls it once when `enable()` starts and then consumes the
-resolved components, block stacks, resident heads, and staged-residency flags
-it returns. Backends do not re-read model declarations.
+The generic model-level and ordinary layerwise paths call it once when
+`enable()` starts and consume its component selection and block stacks.
+Ordinary layerwise offload also respects a resolved `skip_reason`, preserving
+the legacy missing-DiT no-op without placing auxiliary components. Non-block
+DiT and encoder state is placed by resolved block tensor identity, so the
+backend does not rediscover block containers by attribute name.
+
+Two compatibility paths remain outside this completed generic cutover:
+model-level pipelines implementing `SupportsModelCpuOffload` still own their
+custom phase lifecycle (RFC #6648 J4), and distributed layerwise offload still
+reads declarations for nested-submodule staging and loader-owned mmap paths
+(J3). The custom model-level protocol is delegated to before generic plan
+resolution; it must not be subjected to the generic encoder/DiT swap checks.
 
 Pipeline component discovery prefers `SupportsComponentDiscovery` declarations:
 
@@ -102,3 +113,17 @@ remains responsible for transfer, synchronization, and storage ownership.
 
 The diffusion [Offloader module design](../../module/diffusion/offloader.md)
 describes how these feature contracts fit into the larger diffusion runtime.
+
+## Backend acceptance tests
+
+`tests/diffusion/offloader/test_backend_plan_contract.py` covers the two
+behaviors the generic cutover changed. A DiT attribute aliasing a streamed
+block must keep the ring's host residency, so placement cannot follow attribute
+names; the CUDA case fails against the previous name-based placement. Both
+backends must then run with their selector helpers and the model declaration
+made unreadable, proving execution consumes the resolved plan alone.
+
+Selection errors, rollback, residency, and enable/disable cycles stay in
+`test_plan_resolver.py`, `test_layerwise_backend.py` and
+`test_sequential_backend.py`. Model-specific phases and executed tied aliases
+remain the J4 contract suite.