Mark Plugin Feature Lane

Goal

Audit highlight, subscript, and superscript against the editor-behavior docs and push the next feature lane through tests/docs/runtime as needed.

Phases

• Audit existing highlight/subscript/superscript plugin surfaces and editor-behavior docs
• Add or update protocol/parity/spec coverage for the supported mark plugins
• Implement any missing markdown/editor behavior tests or runtime changes
• Verify and summarize next feature status