Refactor fullscreen rejection handling to use WebIDL react pattern

This commit modernizes how the Screen Orientation specification responds to
fullscreen request rejections by using the WebIDL 'react' concept instead
of the previous flag-based approach.

Changes:
- Replace flag-based 'pending fullscreen request flag unset due to rejection' pattern
- Use WebIDL 'react' pattern with 'pending fullscreen request promise'
- Update reference to use proper 'pending fullscreen request flag' terminology
- Extract duplicated descendant document logic into helper algorithm 'reject descendant document orientation promises'
- Remove redundant fullscreen exit handling (already handled by Fullscreen spec)
- Add clarifying notes for algorithm logic and same-origin security restrictions
- Maintain all existing algorithm behavior while modernizing the specification architecture

This enables cleaner, more maintainable specification text that follows modern
WebIDL patterns for promise-based APIs and eliminates code duplication. The change
complements the fullscreen spec changes in whatwg/fullscreen that add both the
exported flag and promise definitions needed for this integration.

Fixes #254
Fixes #255

Author: marcoscaceres

index.html

@@ -926,19 +926,19 @@ <h2>
         requested fullscreen. This prevents cross-origin documents from
         detecting the fullscreen state of their embedding document.
       </p>
+      <aside class="note" title="Fullscreen exit handling">
+        <p>
+          When a [=document=] exits fullscreen, the [[FULLSCREEN]]
+          specification automatically calls the [=fully unlock the screen
+          orientation steps=], which handles unlocking orientation and
+          rejecting any pending orientation lock promises for both the document
+          and its same-origin descendant documents.
+        </p>
+      </aside>
       <p>
-        When a [=document=] |doc| exits fullscreen, the user agent MUST:
-      </p>
-      <ol class="algorithm">
-        <li>Run the [=fully unlock the screen orientation steps=] with |doc|.
-        </li>
-        <li>[=Reject descendant document orientation promises=] for |doc|.
-        </li>
-      </ol>
-      <p>
-        [=React=] to a [=document=] |doc|'s [=pending fullscreen request
-        promise=]. When the promise is [=reject|rejected=], run the following
-        steps:
+        [=promise/React=] to a [=document=] |doc|'s [=pending fullscreen
+        request promise=]. When the promise is [=reject|rejected=], run the
+        following steps:
       </p>
       <ol class="algorithm">
         <li>If |doc|'s {{Document/[[orientationPendingPromise]]}} is not