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the same unpublished .html path, so it would have shown an empty frame.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/halftone-dissolve/README.md

@@ -0,0 +1,68 @@
+# halftone-dissolve
+
+Transition texture primitive: scene A dissolves into scene B through a
+growing halftone dot field. Accent ink dots pop onto a fixed staggered grid
+smallest-first, each dot then opens into a circular window onto B (the ink
+leads the window by a beat, so every site flashes accent before B fills it),
+until the windows merge and B fully replaces A. Radius per dot is a pure
+function of (grid position, progress) via a seeded threshold map; the
+direction enum orders the thresholds. 3.5s authored, elastic HOLD on B
+(never time-scaled).
+
+## Mounting
+
+```html
+<div
+  class="clip"
+  data-composition-id="halftone-dissolve"
+  data-composition-src="./components/halftone-dissolve.html"
+  data-variable-values='{"direction":"center","dot_size":"large","accent":"blue"}'
+  data-start="0"
+  data-duration="3.5"
+  data-track-index="0"
+></div>
+```
+
+## Scene slots
+
+Both scenes are slots. Place inert templates anywhere in the HOST page:
+
+```html
+<template data-slot="halftone-dissolve-a"> ...outgoing scene... </template>
+<template data-slot="halftone-dissolve-b"> ...incoming scene... </template>
+```
+
+Direct `img` / `video` children are stretched to cover the stage; arbitrary
+HTML is centered. A missing slot falls back to a token monogram tile
+(A neutral on `--muted`, B tinted by the accent).
+
+## Variables
+
+| id          | type   | default | notes                                                                                                              |
+| ----------- | ------ | ------- | ------------------------------------------------------------------------------------------------------------------ |
+| dot_size    | enum   | medium  | small, medium, or large grid pitch (host width / 44, 30, 20).                                                      |
+| direction   | enum   | ltr     | ltr sweeps left to right, center grows from the middle, noise is a pure seeded scatter.                            |
+| dissolve_at | number | 1.1     | Seconds from mount start when the dissolve begins. Clamped so the 1.3s dissolve and any exit stay inside the clip. |
+| accent      | enum   | green   | green maps to --brand, blue to --accent, violet to --accent-2. Colors the ink dots.                                |
+| exit        | enum   | none    | none holds scene B (frame roots own transitions); fade or up add a 0.45s departure.                                |
+
+## Choreography
+
+- 0.00s: scene A holds clean; no dots.
+- dissolve_at: the field starts. Dots appear smallest-first in threshold
+  order (with a seeded jitter so the front stays granular, never ruled),
+  each as accent ink, then opening into a window onto B.
+- dissolve_at + 1.3s: the windows merge past full coverage; the clip path
+  releases and B holds clean.
+- HOLD: dead still on B, elastic with the clip duration.
+
+Sync point `dissolve` fires as the field starts; align SFX to it.
+
+## Determinism
+
+The grid and its threshold map are computed once at build from a fixed LCG
+seed. One plain-object progress tween feeds one painter that redraws the
+ink canvas from scratch and rebuilds B's clip-path string, all pure
+functions of progress, so eventful seeks land identical frames in any order
+and either direction. The raster and clip-path basis is the host box
+measured once at mount.