feat(registry): bring back the video-primitive moves (#3169)

Restores the 208 catalog items reverted after their previews 404'd in
production, this time on the payload mechanism rather than the .html files
that caused the outage.

The generator no longer writes a preview document to docs/public. That writer,
and the machinery under it, existed only to produce files the docs host
discards, so it is gone rather than bypassed. Items now embed the composition
itself via a payload, which is what the previous change already does for the
items that were already in the catalog.

The variables explorer is parked, not restored: it drove its preview through
the same unpublished .html path, so it would have shown an empty frame. Items
that declare variables get the live player plus the static variables table, and
reconnecting the explorer to payloads is a follow-up.

registry/components/yt-screen-warp/demo.html

@@ -54,7 +54,12 @@
       #yt-sw-fill {
         position: absolute;
         inset: 0;
-        background: linear-gradient(150deg, #ff8fb8 0%, #7d5cff 60%, #2a7fff 100%);
+        /* Placeholder footage: L3 of the ink over the dark ground, plus a 3px
+           achromatic hatch so the plate reads as a media surface, not an empty box. */
+        background:
+          repeating-linear-gradient(90deg, rgba(255, 255, 255, 0.05) 0 1px, transparent 1px 3px),
+          rgba(255, 255, 255, 0.18);
+        box-shadow: inset 0 0 0 1px rgba(255, 255, 255, 0.14);
         display: flex;
         align-items: center;
         justify-content: center;