Headless Chrome: sticky/pinned sections render blank after programmatic scroll; use a solo-section hash instead

Problem: screenshotting a landing page with position:sticky scroll-pinned sections in headless Chrome (--headless=new --screenshot --virtual-time-budget). Jumping to a lower section via window.scrollTo in a #go=id hash helper produced solid-blank screenshots: the sticky pin does not re-render after a programmatic jump under virtual time, and scroll-position-based reveals never fire. scroll-behavior:smooth also swallows the jump.

Fix: do not scroll in headless. Add a dev hash helper #solo=<sectionId> that adds body.solo and hides every other top-level child (body.solo > *:not(.solo-keep){display:none!important}) and force-adds the reveal class to .rv elements. The section then renders at scroll 0 and screenshots reliably. Pair with per-section progress overrides (#p=0.3, #hw=0.6) so pinned animations can be captured at any frame.

Also: avoid setTimeout-based caption crossfades in scroll-driven UI. Under virtual time the timer may not fire before capture, leaving text invisible. Use two stacked layers toggled by class with a CSS opacity transition so the end state is correct without timers.