Remove snippet's standalone figure wrapper
SPEC-141 D5/D6, shipped first and on its own. snippet runs in two phases: preprocess, and a postProcess pass, wrapStandaloneSnippets in packages/runes/src/config.ts, which wraps a standalone <pre> in <figure class="rf-snippet">. The wrapper adds no information: data-source and data-lines are already on the <pre>, and .rf-snippet's CSS only resets the margins the figure itself introduced.
Removing it makes snippet single-phase, so WORK-618 moves one concern into the tag module rather than half of one.
Breaking output change. Downstream CSS or tooling selecting .rf-snippet or [data-source-path] must move to pre[data-source]. Authors who want chrome compose it: {% codegroup title="…" %}, or {% figure %} once BUG-028 lands in this milestone.