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.
When no CUDA/ROCm/XPU GPU is available, route diffusion load/generate to the
native stable-diffusion.cpp engine instead of diffusers, with diffusers as the
guaranteed fallback. On CPU sd.cpp is 1.4-2.8x faster and uses 1.5-2.2x less RAM.
- diffusion_engine_router: centralised engine selection (built on the existing
select_diffusion_engine), env opt-outs, MPS gating, recorded fallback reason.
- sd_cpp_backend (SdCppDiffusionBackend): the diffusers backend method surface
backed by sd-cli, with lazy binary install, registry-driven asset fetch,
step-progress parsing, and cancellation.
- diffusion_families: per-family single-file VAE + text-encoder asset mapping.
- sd_cpp_engine: cancellation support (process-group kill + SdCppCancelled).
- routes/inference + gpu_arbiter: drive the active engine via the router; the
API now reports the active engine and any fallback reason.
- tests for the backend, router, route selection, and cancellation.
