fix(core): parent-frame proxy bypass, data-preload-eager opt-out, configurable threshold

- Player: _adoptIframeMedia now skips media with preload="metadata" or
  "none", preventing parent-frame proxies from bypassing the preloader.
  MutationObserver extended to watch preload attribute changes so proxies
  are created just-in-time when the preloader promotes a clip.

- init.ts: lazy-mode demotion loop skips elements with data-preload-eager,
  letting power users keep specific clips eagerly buffered.

- mediaPreloader: reads window.__HF_LAZY_PRELOAD_THRESHOLD as an override,
  falling back to the default 6.

packages/core/src/runtime/init.ts

@@ -1263,6 +1263,10 @@ export function initSandboxRuntimeModular(): void {
       // timed clips and would never promote them back.
       for (const mediaEl of mediaEls) {
         if (!mediaEl.hasAttribute("data-start")) continue;
+        // Power-user opt-out: data-preload-eager keeps a clip eagerly buffered
+        // even under lazy mode, useful when a specific clip must be instantly
+        // available regardless of playhead proximity.
+        if (mediaEl.hasAttribute("data-preload-eager")) continue;
         if (mediaEl.preload === "auto" || mediaEl.preload === "") {
           mediaEl.preload = "metadata";
           // Kick off the metadata fetch explicitly — some browsers (Chrome Lite