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the same unpublished .html path, so it would have shown an empty frame.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

@@ -0,0 +1,38 @@
+# line-swap
+
+A masked full-line beat replacement. `line_a` rises into an overflow-hidden mask and holds center; on the swap beat it exits up through the mask as `line_b` enters bottom-up on the same beat (single restrained slam, smooth ease, no overshoot). Optionally an accent underline draws left to right beneath `underline_word` of `line_b` after it lands. Generalizes flagship frame 01.
+
+3.5s authored, elastic HOLD, exit `none` by default.
+
+## Variables
+
+| id               | type   | default                               | notes                                                                                                                     |
+| ---------------- | ------ | ------------------------------------- | ------------------------------------------------------------------------------------------------------------------------- |
+| `line_a`         | string | `Everyone promised you AI.`           | Holds center, exits up on the beat.                                                                                       |
+| `line_b`         | string | `Almost nobody promised you control.` | Enters bottom-up on the beat, then holds. Font size auto-fits to the longer line.                                         |
+| `swap_at`        | number | `1.5`                                 | Beat second from mount start. Clamped so line A fully arrives first and the swap plus underline complete before any exit. |
+| `underline_word` | string | `control`                             | First case-insensitive substring match in `line_b` gets the underline. Empty or unmatched disables it.                    |
+| `accent`         | enum   | `green`                               | Underline color: green rides `--brand`, blue rides `--accent`, violet rides `--accent-2`.                                 |
+| `exit`           | enum   | `none`                                | `none` holds until the cut; `fade` departs opacity-only; `up` fades while rising.                                         |
+
+## Mount
+
+```html
+<div
+  class="clip"
+  data-composition-id="line-swap"
+  data-composition-src="./line-swap.html"
+  data-variable-values='{"line_a":"Everyone ships demos.","line_b":"Almost nobody ships proof.","underline_word":"proof","swap_at":1.3,"accent":"blue"}'
+  data-start="0"
+  data-duration="3.5"
+  data-track-index="0"
+></div>
+```
+
+Elastic root: no `data-width`/`data-height`; it fills whatever box the host clip gives it. Timeline registers under the literal `line-swap` key.
+
+## Notes
+
+- Both lines are nowrap and share one deterministic character-count font fit, so the swap never reflows.
+- The underline is the flagship scaleX bar draw (left transform-origin, explicit 0 to 1 endpoints), hidden until the cue and seek-safe in both directions.
+- The hold after the underline lands is completely still; frame roots own transitions.