Schema: allow "@component" on metadata elements and "docinfo"

[rbeezer]

Reported on pretext-support (VS complaining about component in subtitle, but all works well): placing component="..." on a subtitle draws a validation complaint, while processing is perfectly happy. The version machinery in assembly acts on any element carrying @component, and the attribute survives into the assembled tree, so the schema is the only gatekeeper — for editor validation (pretext-tools) and for -V alike.

Two commits extend the allowance where a version-conditional variant makes sense:

1. Title-like metadata elements — title (both forms), subtitle (both forms, the reported case), shorttitle, plaintitle, creator, caption, attribution. A caption reading "the red curve" needs a variant in a black-and-white version; a title's companions travel with it.
2. docinfo — the version mechanism's motivating example (per-version latex-image-preamble color palettes) is authored as two docinfo elements, yet @component was not allowed there.

Precedents: idx already carries the allowance, and commentary requires the attribute.

Verified: @component planted on one instance of each of the eight elements in the sample article draws exactly eight complaints from the current schema and zero from the revised schema (the total error count drops by exactly eight, nothing else moves); regenerating .rnc/.rng from unmodified source was checked clean first, so each commit pairs the literate source with its derived files.

Claude Fable 5, acting as a coding assistant for Rob Beezer