The prepareFlattenedInnerRoot function creates a wrapper div when
inlining sub-compositions. This wrapper had no width/height, which
broke CSS height:100% chains — any sub-composition using percentage
heights with flexbox centering would collapse to 0px and render
content at the top instead of centered.
Read data-width/data-height from the inner root and set matching
pixel dimensions on the wrapper's inline style. Applied in both the
compiler (server-side bundling) and the runtime (browser-side
composition loader).
Adds a producer regression test with a centered card sub-composition
that fails without this fix.
