v0.81.0: one chassis, N themes — the chassis split, three verified theme ports, the renderer plugin seam

The chassis release: the showcase splits into a genre-free chassis (delivery plumbing, the
token system, the prose foundation, composition primitives, documented seams with
per-element removal notes) and Waymark, now an explicit theme over it. Three verified
example themes land beside it, each an MIT-attributed port of a newly popular external
theme, glance-verified against its original by fresh-context visual review: AstroPaper
(the pure theme-seam proof), Foxi (the composition stress, with the production-verified
themed 404 the chassis now documents as a pattern), and hugo-theme-gallery (the media
stress, the upstream's own justified-layout geometry).

The engine gains the renderer's remarkPlugins/rehypePlugins seam, table scrolling by
default (opt-out via tableScroll: false), sitemap extraRoutes plus the unlistedRoutes
build check, CairnHead's titleTemplate, and a computed-margin regression test for the
prose flow rhythm (whose specificity bug this window also fixed). The type scale no longer
compounds with the ultra-wide root clamp.

No consumer action required: every change in the window is additive, and existing sites
build unchanged. Editor-facing: the suggestion popover and editor accessibility work from
0.78.2 carries forward unchanged; the admin surface is untouched this window.