case-order=upper-first/lower-first makes the string comparison
case-insensitive, with case used only to break ties among otherwise-equal
strings; without it the default codepoint order is unchanged. Part of #82.
A literal result element and its attributes in a stylesheet namespace
bound by stylesheet-prefix are rewritten on output to the namespace and
prefix bound by result-prefix (#default supported), so a stylesheet can
emit literal xsl: elements. Prefixes resolve through the in-scope xmlns
declarations; aliases fold across include/import. Splits AVT parsing into
XSLTAVT.swift and trims the transformer to stay within length budgets.
Only the static deduplicated helper moves to XSLTElements.swift (it needs
no private types); attributeSetAttributes stays in the transformer so the
evaluation context and result-item types remain file-private, satisfying
the one-file-scope-type namespacing gate.
Named attribute sets apply to a literal result element or xsl:element via
use-attribute-sets; sets may include other sets recursively with a cycle
guard, set attributes are lowest precedence so the element's own
attributes override them, and same-named attributes collapse to the last.
Folded across xsl:include/xsl:import. Splits the element-building helpers
into XSLTElements.swift to stay within the file-length budget.
