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.
9a2cc341c5