fix(home): hide the demo bundles' unpack placeholder

[cevheri]

Opening the site flashed a blue bolt on a dark card inside the demo frame before the walkthrough appeared. That is the bundler's own unpack placeholder, #__bundler_thumbnail — a #1d4ed8 rounded rect with a white lightning polygon — shown until the bundle replaces the whole document with the real page. The Unpacking... pill in the corner is the same thing.

Both bundles now hide the two elements from their own head style:

#__bundler_thumbnail, #__bundler_loading { display: none !important; }

Our wrapper already paints the box, so the frame simply stays empty for those few frames instead of flashing an unrelated icon. Nothing in the unpacking script reads or writes either element, and the injected rule lives in the placeholder document's head — it disappears along with it when documentElement.replaceWith swaps in the real page, so it cannot affect the walkthrough itself.

Measured, before and after

Sampling a cold frame every 5ms and reading the placeholder's computed style:

	placeholder visible
before	yes, from ~15ms until the document swap
after	never

Both clips still unpack and play: desktop @1440px and phone @390px each reach the real document and keep advancing.

Note

These are vendored artifacts, so the rule carries a comment saying to re-apply it if the bundles are regenerated. Worth folding into the generator instead, if it is easy on that side.

Gate clean: 0 errors, prettier clean, oxlint clean, knip clean, 75 tests pass. The rule ships in dist/demo/ for both bundles.