Show emissive meshes in the Light Group panel

[riouxr]

Fixes #13.

The reported bug

Emissive materials never appeared in the Light Groups panel. Cycles assigns emissive meshes to light groups like any other emitter, but LightGroup.py filtered every list on obj.type == 'LIGHT', so they were excluded by construction — from the group lists, Assign/Unassign, Deselect All, solo, and group removal.

All of those now go through one helper that resolves lights and emissive meshes.

A second bug found while fixing it

is_emissive_node_active() was added in 702d252 ("emission at 0 or with color at black don't show up in list anymore") and then deleted, along with its call site, by 8d679a8 ("cleanup code").

Without it, node type alone decided whether a material was emissive — and every Principled BSDF carries an Emission Strength socket, so every ordinary material qualified. It is restored and now shared by both panels, so the Light Editor and Light Group lists agree on what counts as emissive.

Other changes in this branch

• Solo restore iterates the visibility backup instead of rescanning the scene. A mesh whose emission is edited away while a group is soloed previously fell out of the scan and stayed hidden with no way back.
• Emissive lookup is cached per material and invalidated from a depsgraph_update_post handler on Material updates only — node-tree walks stay out of the redraw path, and object transforms no longer force a full rescan.
• Removed a module-level execute() and _disable_material_node() from LightEditor.py. Both had been dedented out of their classes and were unreachable; nothing referenced either name.
• blender_manifest.toml still read "Adds buttons to quickly switch between mesh display types" and tagged itself "Mesh". That is the text Blender shows in the extensions list.

Verification

Headless suite against Blender 5.1.2, all passing:

Check	Result
Add-on registers / unregisters cleanly	pass
Emission strength 5 listed; strength 0 and black-colour not listed	pass
Default Principled BSDF not listed	pass
Principled with emission turned up listed	pass
Emission behind a Mix Shader found (recursion)	pass
Empty material slot does not raise	pass
Cache refreshes after a shader edit	pass
Assign / Unassign round-trip on an emissive mesh	pass
Solo hides non-members, keeps members	pass
Unsolo restores the user's own pre-existing hidden state	pass

Icon identifiers used in the new rows were validated against UILayout.bl_rna.

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