Lights should have an ID

Similar to surfaces and volumes, (at least area) lights should have an ID to populate the `objectId` frame channel from. That would allow apps to select area lights, just as some apps support that for surfaces and volume object types.

Start by reviewing the existing documentation for surfaces, volumes, area lights, and the objectId frame channel. Compare how IDs are documented for surfaces and volumes; the work is done when area lights, at minimum, have an ID that can populate objectId and support application-side selection.

Written by the indexing model from the issue text.