Route Napari components by layer mode

[trissim]

Outcome

Use the correct Napari vocabulary when component values create separate viewer layers.

What changed

• make the Napari route-key owner consume ViewerComponentMode.LAYER
• rename Napari-specific route tests and fixtures from slice to layer
• preserve the generic/Fiji SLICE path for actual ImageJ Z slices

Depends on the declaration merged in OpenHCSDev/ZMQRuntime#2. No package version or release is included here.

Verification

PYTHONPATH=src:../zmqruntime/src ../../.venv/bin/python -m pytest tests/test_streaming_receiver_core.py -q
13 passed

PYTHONPATH=src:../zmqruntime/src ../../.venv/bin/python -m ruff check --select E9,F63,F7,F82 src/polystore/streaming/receivers/napari/layer_key.py tests/test_streaming_receiver_core.py
All checks passed