Schema documents that form an import chain are now compiled as a single
union (Document.init(composing:)) rather than as separately compiled
documents merged after the fact. Merging froze each document's content
models with only the substitution-group members visible in its own import
closure, so a substitution member declared in an importing document never
reached the head's content model in the imported document.
This clears XSTS ctZ007, a cross-document substitution group, taking
valid-instances-rejected (#146) from 4 to 3 with no other bucket moving
(now 1/262/3/147). The union pools each document's container closure into
one context, deduplicating a document supplied directly and also reached
through another's import by target namespace and component-name set.
The XSTS runner uses the union only for import chains, detected by matching
import/include/redefine schemaLocation basenames against the listed
documents; independent roots stay on the lenient per-document merge, which a
union would wrongly trip with cross-document checks.
The remaining three valid-instance rejections are the Ethiopic-digit \d
cases (reS17, reS38, reZ004v), which contradict \d = \p{Nd}.
Mihaela Mihaljevic
4df9eda52e