[w3c/screen-orientation] Add hidden document monitoring in parallel orientation lock steps (PR #272)

The 'apply orientation lock' algorithm monitors if documents stop being fully active during parallel execution, but was missing equivalent monitoring for document visibility state changes.

This creates inconsistency where:
- Common safety checks prevent hidden documents from starting locks
- Parallel steps monitor fully-active state changes
- But parallel steps don't monitor hidden state changes

This fix adds monitoring for documents becoming hidden during parallel orientation lock processing, with consistent AbortError rejection.

Fixes #256

The following tasks have been completed:

 * [ ] Modified Web platform tests (link to pull request)

Implementation commitment:

 * [ ] WebKit (https://bugs.webkit.org/enter_bug.cgi)
 * [ ] Chromium (https://bugs.chromium.org/p/chromium/issues/detail?id=)
 * [ ] Gecko (https://bugzilla.mozilla.org/show_bug.cgi?id=)


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You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/screen-orientation/pull/272

-- Commit Summary --

  * Add hidden document monitoring in parallel orientation lock steps

-- File Changes --

    M index.html (5)

-- Patch Links --

https://github.com/w3c/screen-orientation/pull/272.patch
https://github.com/w3c/screen-orientation/pull/272.diff

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