feat(render): XML emitter and schema

Element-per-field; attributes carry identity only and there is no mixed
content, so a consumer's XPath never has to distinguish the two.

Schema validation is an opt-in make check-schema via xmllint, not an
in-suite assertion: validating XSD needs an XML library and the
dependency list is frozen. It prints SKIPPED loudly when xmllint is
absent, because a silent skip reads as a pass. The suite asserts
well-formedness properties directly instead.

This corrects the design spec, which claimed in-test validation.

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