[mobileWallLayout] Add play-on-visibility and ordered loading

[speckofthecosmos]

Summary

Two mobile-first behaviors on top of the existing layout fix, and a
display-name rebrand to scrollFeed to reflect the broader feature set.

• Play-on-visibility. An IntersectionObserver plays each <video> at
  10% visibility and pauses it when it leaves the viewport. Concurrent
  playback caps at 2–3, which keeps iOS Safari comfortably under its
  ~20-simultaneous-<video> decoder ceiling. On a 20-card wall page
  (Stash's minimum per-page size), the unchanged plugin leaves every
  preview autoplay firing at once — this PR fixes that.
• DOM-ordered load queue. On page mount, Stash fires parallel fetches
  for every <video src>. Over cellular that splits the uplink 20 ways
  and no video is playable for many seconds. The queue cancels those
  fetches via removeAttribute('src') + video.load() and re-issues them
  top-down, 2 at a time. Advances on canplay or a 500ms fallback — the
  top clip is playable fast on cellular, and the whole page is in-flight
  within ~5 seconds so moving into a weaker-signal area doesn't leave the
  bottom of the page with zero bytes.
• Rebrand. Display name in the YAML is now scrollFeed with an
  updated description. The plugin ID stays mobileWallLayout so existing
  installs upgrade cleanly.

Internals

• File wrapped in an IIFE to keep top-level identifiers out of global
  scope (Stash loads all UI plugins into the same document).
• URL matching uses window.location.pathname (exact match on /images
  and /scenes/markers) rather than href.includes.
• Stale-timer safety: beginLoading captures the current _loading Set
  in its closure so any setTimeout advance that fires after
  deactivateVideoBehavior nulls plugin state detects the mismatch and
  no-ops.
• All non-iteration references use WeakMap/WeakSet so video GC isn't
  blocked when React unmounts elements.

Tuning knobs

Constants at the top of the load-queue section:

Constant	Default	Effect
threshold (IntersectionObserver)	0.1	Lower = more continuous scroll; higher = stricter per-clip focus.
_MAX_CONCURRENT_LOADS	2	Higher = full-page finishes sooner; lower = top clips get more bandwidth.
_LOAD_ADVANCE_MS	500	Short = tail starts fetching sooner; long = top clips get more solo time.

Test plan

• iPhone 16 Pro Max against a home-NAS Stash instance over remote
  cellular: 20-card marker wall plays 2–3 concurrent, top video canplay
  in ~1s, whole page in-flight by ~5s, scroll to video renamerOnUpdate: segment config options into different file #18 shows bytes
  already accumulating.
• Airplane-mode mid-scroll: already-loaded clips continue playing;
  entering a not-yet-loaded clip shows a queued state and resumes once
  back online.
• Navigate away from /scenes/markers and back: observers tear down
  and re-attach cleanly, no leaked style tag or decoder stampede.
• validator/validate.js passes locally; YAML schema-valid.

Internal plugin ID is unchanged (mobileWallLayout), so users with the
current published plugin will transparently upgrade.

[speckofthecosmos]

Apologies — the scrollFeed rebrand silently dropped the discourse URL from two places: the url: field in mobileWallLayout.yml was repointed to a GitHub tree URL, and the discourse link was removed from the top of the README. That breaks the convention every other plugin in this repo follows (the url: field is what the Stash Plugins panel surfaces as the support link), and it was my oversight.

To be upfront: this PR was drafted with AI assistance (Claude Opus 4.7 — noted in the first commit's Co-Authored-By trailer). The rebrand/rewrite step is where the discourse URL was silently dropped, and I did not catch it on my review.

Just pushed b6ad2e3 restoring both:

• mobileWallLayout.yml → url: https://discourse.stashapp.cc/t/mobile-wall-layout/6160
• README → Discussion: link at the top

Adding a pre-PR check to my own routine so the url: field gets verified against the discourse convention before any future CommunityScripts plugin PR I open.

[discourse-stashapp]

This pull request has been mentioned on Stash Forum. There might be relevant details there:

https://discourse.stashapp.cc/t/mobile-wall-layout/6160/3