Adds docs/render with a handover covering what shipped on this branch,
the verification methodology, build gotchas, and the prioritized list of
remaining work, alongside the phase plan and the index of the Gecko
layout study that informed it.
Adds render-repros: one HTML fixture per bug fixed here (absolute fills,
mixed inline+block layout, carousel clipping, z-index order, clear,
tables, image density and alt fallback) plus run.sh to render them all
in obscura and Chromium for side-by-side checking. Run before and after
any layout or paint change.
