feat(core): add loop and data-playback-rate for media elements

- data-playback-rate: per-element slow-mo/fast-forward (0.1-5x range)
  Multiplied with global transport rate. Affects timeline duration
  calculation when source duration is used as fallback.

- loop: native HTML loop attribute now works correctly in the runtime.
  Wraps media playback from mediaStart when source reaches end.
  Enables looping short clips over longer durations.

Both follow the existing data-media-start/data-volume pattern.

packages/core/src/lint/hyperframeLinter.ts

@@ -406,7 +406,8 @@ export function lintHyperframeHtml(
         code: "base64_media_prohibited",
         severity: "error",
         message: `Inline base64 audio/video detected (${(dataSize / 1024).toFixed(0)} KB)${isSuspicious ? " — likely fabricated data" : ""}. Base64 media is prohibited — it bloats file size and breaks rendering.`,
-        fixHint: "Use a relative path (assets/music.mp3) or HTTPS URL for the audio/video src. Never embed media as base64.",
+        fixHint:
+          "Use a relative path (assets/music.mp3) or HTTPS URL for the audio/video src. Never embed media as base64.",
         snippet: truncateSnippet((b64Match[1] ?? "").slice(0, 80) + "..."),
       });
     }