Composition reliability acceptance fixture

Compact, media-free Studio project for validating composition editing as one stack:

two root hosts reuse title-card.html at different times;
nested-shell.html hosts the same title card one level deeper;
the title card uses a transparent overflow mask around an editable headline;
adjacent clips on one track provide a clean collision/new-track drop target;
a cross-track overlap exercises normal visual layering.

Copy this directory to scratch before browser acceptance. Exercise open, composition insert, single/multi move, collision placement, overlap layering, cut, headline color/font-size, and one-step undo. The checked-in fixture must remain unchanged.