Extension Refactor Tracker

DoneURL copied

• Task
  Add real extension contract in vyasa/extensions.py

• Task
  Switch built-ins to EXTENSION.register(app)

• Task
  Remove markdown hard-coded fence fallbacks

• Task
  Move Mermaid/D2/Cytograph/Tasks/Cryptograph render ownership into extension modules

• Task
  Move bookmarks and annotations route/storage wiring out of core.py

• Task
  Move search and RBAC admin route registration behind extensions

• Task
  Serve slide assets from extension-owned URLs under /static/extensions/...