Derive the docs staging css/docinfo fan-out from the staged tree

docs/pom.xml carries a hand-maintained per-directory roster for the staged site’s assets: one <copy> of css/ and the two docinfo-*.html theme files per staged directory, every entry failonerror="false". The roster is a copy of a fact the staged tree already states (which directories contain rendered pages), and a missed entry fails nothing: the AsciiDoctor error handler does not see it, the ant-level signal is suppressed, and the first report is a human noticing a published page with no stylesheet and no site nav. Every new docs directory (the principles section, per the principles-section-under-architecture item, is the occasion that exposed this) must remember to append two entries by hand.

The fix is to derive the fan-out: a build step that walks the staged tree and copies css/ and the docinfo files into every staged directory containing a .adoc, deleting the roster. SchemaIdentifierDriftCheck reading the booted store instead of the DDL is the in-repo precedent for reading the tree rather than listing it; the roadmap-tool staging step already runs Java over the same tree, so it is a natural host.