Each .code-example block is now collapsed by default: the header keeps
its title and copy button and gains a chevron toggle (built at runtime
by code-examples.js, works identically in vanilla and Vue demos since
they share the demo bundle). Expanded state is one next-frame class
flip with aria-expanded/aria-controls on the button.

Demo page height drops 50555px -> 32603px (~35% shorter); demo-sync
stays at 0 differences.