Landing: dual-pitch overhaul — showcase, editor proof, objections, trust creed

docs/architecture/05-rendering-theming-preview.md

@@ -576,6 +576,14 @@ It does two deterministic things, on a **deep copy** of the spec:
    and **removes** `width`; Full sets both — see spec §04 → Fit-mode sizing),
    recursing the same way.
 
+The fit recursion has one Vega-Lite limit: **`"container"` sizing only works on
+single and layered views** — facet children fall back with a warning (panel
+widths become timing-dependent) and concat children fall back to pad-autosize
+(axes overflow a fixed card). A surface that renders a composed spec at a fixed
+size must ask for `fitMode: 'default'` and let the spec's declared per-view
+`width`/`height` stand — the landing's `LandingChart` exposes this as a prop
+for its composed demos.
+
 This is _content_ preparation, not embedding, and it is fully covered by the
 _Live Preview_ spec. The only invariant this doc cares about: