fix(lint): drop false media_in_subcomposition rule (#2765)

The media_in_subcomposition rule blanket-errored every <video>/<audio>
inside a sub-composition, claiming nested media is "never seeked/decoded
and renders blank/black". This is false: the runtime discovers media with
a flat document.querySelectorAll("video, audio"), resolves each element's
host composition via closest("[data-composition-id]"), and rebases its
local data-start by the accumulated absolute start of every ancestor
composition (packages/core/src/runtime/{media,startResolver}.ts). Media
seeks and decodes at any nesting depth, verified end to end through the
producer render path.

- Remove the rule and flip its test to assert nested media is NOT flagged.
- Drop the now-dead media_in_subcomposition clause from the registry
  components test.
- Drop the equivalent pre-render guard from the faceless-explainer and
  pr-to-video assemble scripts.
- Correct the reference docs (hyperframes-core SKILL, data-attributes,
  variables-and-media, composition-patterns; hyperframes-cli
  lint-validate-inspect): media works at any depth. Preserve the one real
  constraint, that a sub-comp timeline cannot reach host-root elements, so
  host-root media motion is authored on the main timeline.

packages/cli/src/registry/registryComponents.test.ts

@@ -26,9 +26,7 @@ async function invalidInstallableMedia(entryName: string): Promise<string[]> {
       isSubComposition: true,
     });
     for (const finding of result.findings) {
-      if (finding.code !== "media_in_subcomposition" && finding.code !== "media_missing_src") {
-        continue;
-      }
+      if (finding.code !== "media_missing_src") continue;
       invalidMedia.push(`${entryName}/${file.path}: ${finding.code}`);
     }
   }