The renderer now waits for all sub-composition timelines to be
registered in window.__timelines before starting frame capture.
Previously only window.__hf root readiness was checked, causing
blank frames when sub-compositions use async data loading (fetch)
or when the headless renderer starts capturing before scripts
complete.
Adds pollSubCompositionTimelines() to both screenshot and
beginFrame render paths, with a diagnostic warning listing which
composition IDs are missing if the timeout expires.