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the same unpublished .html path, so it would have shown an empty frame.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/top-down-letters/registry-item.json

@@ -0,0 +1,72 @@
+{
+  "$schema": "https://hyperframes.heygen.com/schema/registry-item.json",
+  "name": "top-down-letters",
+  "type": "hyperframes:component",
+  "title": "Top-Down Letters",
+  "description": "Letters descend from above in a pronounced staircase with zero blur.",
+  "tags": ["video-primitive", "motion-primitive", "remocn-port", "typography"],
+  "files": [
+    {
+      "path": "top-down-letters.html",
+      "target": "compositions/components/top-down-letters.html",
+      "type": "hyperframes:snippet"
+    }
+  ],
+  "variables": [
+    {
+      "id": "text",
+      "type": "string",
+      "role": "content",
+      "label": "Text",
+      "description": "Word that cascades in. Every character becomes one letter span, in source order.",
+      "default": "CASCADE"
+    },
+    {
+      "id": "distance",
+      "type": "enum",
+      "role": "motion",
+      "label": "Distance",
+      "description": "Multiplies the drop the timeline drives, so the letters cover a shorter or longer run in the same time.",
+      "default": "standard",
+      "options": [
+        {
+          "value": "close",
+          "label": "Close"
+        },
+        {
+          "value": "standard",
+          "label": "Standard"
+        },
+        {
+          "value": "far",
+          "label": "Far"
+        }
+      ]
+    },
+    {
+      "id": "tone",
+      "type": "enum",
+      "role": "style",
+      "label": "Tone",
+      "description": "Letter colour: ink for light frames, paper for dark ones, accent rides --brand.",
+      "default": "ink",
+      "options": [
+        {
+          "value": "ink",
+          "label": "Ink"
+        },
+        {
+          "value": "paper",
+          "label": "Paper"
+        },
+        {
+          "value": "accent",
+          "label": "Accent"
+        }
+      ]
+    }
+  ],
+  "preview": {
+    "poster": "https://static.heygen.ai/hyperframes-oss/docs/images/catalog/components/top-down-letters.png"
+  }
+}