add a hash-override to screenshot scroll-driven animation states

Headless Chrome cannot scroll, so scroll-pinned / scroll-progress animations always render at their initial state in --screenshot, making it impossible to verify mid/end frames. Fix: have the scroll handler check location.hash for a forced progress value (e.g. match /p=([0-9.]+)/) and use it instead of the computed scroll progress when present. Then screenshot the section in isolation at file://...#p=0, #p=0.5, #p=1 to capture any keyframe. Also gate the section so at max-width it becomes position:static (unpins) and the animated pieces reset transform, so mobile just stacks. Leave the hash hook in during dev; it is harmless in prod (no hash = normal scroll).