Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Design sketches for four improvements:
- sherlodoc mld page indexing
- custom inline extensions in odoc
- @page-tags producer and consumer
- @figure native block-tag plugin

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

- layer.mld: add Layer module, update description for new core type
- batch.mld: add Profile and Snapshot modules
- doc.mld: add Doc_build module
- generate.mli: add doc comment for run function
- patches.mli: new interface file for Patches module

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>