Distinguish viewer layers from image slices

[trissim]

Outcome

Add the missing shared viewer placement term for components that route to separate Napari layers.

What changed

• add ViewerComponentMode.LAYER = "layer"
• retain ViewerComponentMode.SLICE = "slice" for genuine Fiji/ImageJ Z-slice placement
• add a contract regression proving the two meanings remain distinct

Verification

PYTHONPATH=src ../../.venv/bin/python -m pytest tests/test_viewer_protocol.py -q
4 passed

PYTHONPATH=src ../../.venv/bin/python -m ruff check --select E9,F63,F7,F82 src/zmqruntime/viewer_protocol.py tests/test_viewer_protocol.py
All checks passed

No package version or release is included.

[Copilot]

Pull request overview

This PR clarifies viewer component placement semantics by introducing an explicit layer placement mode for components that map to separate Napari layers, while preserving slice specifically for Z-plane (Fiji/ImageJ-style) placement.

Changes:

• Add ViewerComponentMode.LAYER = "layer" to the shared viewer protocol enum.
• Preserve ViewerComponentMode.SLICE = "slice" as a distinct mode for plane/Z-slice placement.
• Add a regression test asserting the two wire values remain distinct and stable.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/zmqruntime/viewer_protocol.py	Adds the LAYER enum value to the shared viewer component placement modes.
tests/test_viewer_protocol.py	Adds a contract/regression test asserting the layer vs slice wire values.

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[augmentcode]

^{PR Risk Analyzer Agent🛡️}

Risk Analysis 🛡️ · Low Risk

✅ Approved — Low-Risk Change

Details

Additive change: adds one enum member `ViewerComponentMode.LAYER = "layer"` and a test asserting `LAYER` and `SLICE` keep distinct wire values. No runtime dispatch enumerates the full member set — modes resolve by string value via `viewer_component_mode_value`, and unrecognized modes are handled gracefully in `component_mode_groups`. No existing tests were altered and no sensitive areas are touched.

[augmentcode]

^{Deep Code Review Agent🐛}

Review completed with 0 suggestions.