Fix discovery and video fallbacks for 0.26.1

[PaulFidika]

What changed

• recreate the PyAV output container when per-request NVENC open fails before falling back to libx264
• remove transient missing-heavy-dependency stubs from sys.modules after each import so later optional find_spec() probes stay honest
• release as gen-worker 0.26.1

Root causes

PyAV retained a failed hardware stream in the original container, so mux startup retried it even after a software stream was added. Discovery similarly retained a synthetic flash_attn module long enough for Transformers to misclassify it as installed while distribution metadata correctly had no entry.

Validation

• uv run --locked --extra dev pytest tests/ -q — 1075 passed, 23 skipped
• focused video/discovery tests — 40 passed
• uv run --locked --extra dev mypy src/gen_worker — 0 errors
• focused Ruff — clean
• uv build — wheel and sdist built
• real image-LoRA DiffSynth -> PEFT -> Transformers 5.5.3 discovery — passed on torch 2.13.0+cu130 / CUDA 13.0