RFC-8 revision 2

Proposed revisions to OME-Zarr RFC-8 (Collections and Extensibility), Fall 2026.

• rfc8_revision_changes.md: all changes to RFC-8, for updating the RFC text
• examples/: example metadata for a collection and a multiscale
• slides/: the presentation (rfc8_revisions.pptx) and the script that generates it (node build.js, requires pptxgenjs)
• upstream/rfc8.md: the current RFC-8 text from ome/ngff main (d844a6a)