day11/doc: remove build_hash_universe, use doc_dep_hashes consistently
The link phase and link DAG node construction now use doc_dep_hashes
(the wider set including {post} and x-extra-doc-deps) instead of
build_hash_universe (all packages in the same solution). This is more
precise — the link phase should only see dependencies it actually
needs for cross-referencing, not the entire solution.
Removes the build_hash_universe hashtable and universe_hashes parameter
from link_package and doc_all_package.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>