feat: add 3 new variants per category with 6-way rotation

Posts: dossier (tab protrusion, ruled paper), manifest (shipping label,
barcode), terminal (CRT green, cursor blink).

Shots: thermal (infrared overlay, temp scale), contact (film sprocket
holes), specimen (lab slide mount).

Bsky: comm ping (chevron, radar rings), torn fragment (jagged top),
broadcast (TV static frame, channel badge).

All categories now rotate through 6 variants. Demo page updated with
comparison tiles for every new variant.