Defines the host-owned policy input to site projection.

The callback returns inert page declarations keyed by qualified collection document IDs, plus optional navigation, redirects, locale policy, and site metadata. This is where a host chooses taxonomy, public routes, and exact source or edit links when its forge has provider-specific URL rules. DocShell then joins those declarations to source content and derives digests, headings, internal links, reading flow, search records, and capability requirements.

A source must not render content, write files, authorize a request, or fetch a collection. It receives already loaded collections specifically so those side effects remain outside the presentation contract.