Two ISO Schematron gaps, the most validator-relevant first:
- Dynamic messages: an assert/report message is now a template, so
<value-of select=> renders the string value of its XPath at the context
node and <name> renders the node's name. A finding can report actual
values ("Age 120 must be under 100") instead of static text.
- Phases: <phase id><active pattern=> scopes which patterns run.
validate(_:phase:) honors a requested phase or the schema's
defaultPhase; nil or #ALL runs every pattern.
let variables, abstract rules/extends, diagnostics, and key()/document()
remain (the last depends on XSLT-style function injection).
Part of #80
