Rebase template-content DOM tree fix from #264 onto current master

[Copilot]

PR #264 was behind master, which blocked merge. This PR replays the same change set on top of current master so the fix can be reviewed and merged against the current base.

• Rebased change set

  • Cherry-picked the original Fix template contents participating in the DOM tree commit onto latest master.
  • Preserved scope and intent of Fix template contents participating in the DOM tree #264 while updating commit ancestry.

• Template-content DOM integration

  • Carries forward the added HTML5 DOM wrappers and template-content handling (TemplateContents and related DOM classes).
  • Keeps parser and serializer updates that make <template> contents participate correctly in the DOM lifecycle.

• Behavioral intent retained

  • <template> content handling remains aligned with the original fix, including parsing/cloning/serialization paths touched by Fix template contents participating in the DOM tree #264.

$dom = $html5->loadHTML('<template><div id="inner">x</div></template>');
$template = $dom->getElementsByTagName('template')->item(0);

// content is represented as a managed fragment participating in DOM operations
$fragment = $template->content;
$node = $fragment->firstChild; // <div id="inner">x</div>