Media sources wait until the node is in the page; thumbnails use the cache.

Every render builds a fresh tree to diff against the live one, and a media
element starts downloading the moment its src is set, even off the
document: a sixteen-second feed scroll built 204 <video> and 1,416 <img>
elements whose loads the morph then threw away — 375 cancelled video
range requests, and full re-downloads from hosts that send no cache
headers. h() now puts the source on data-lazy-src and it becomes src only
once the node is in the page (render promotes it; a MutationObserver covers
nodes appended by hand); the morph treats the two as one attribute.

The thumbnail maker fetched every original with cache: 'reload' (38 MB in
one scroll); the face was just painted from that URL, so the cache serves.
holdScroll now recovers an anchor row that a large insert above pushed out
of the window, from the height cache. Tests: the YouTube test accepts the
in-view players autoplay loads; the stability test watches the notice for
the catch-up, since the new rows land inside the window's spacer.

Co-Authored-By: Claude Fable 5.1 <[email protected]>
Claude-Session: https://claude.ai/code/session_01VhUKyXKtsJ67QAFfb7QR9D

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