Order nested document visibility and unload teardown #581

Parent: #267. Webview epic: #264.

Problem

After an allowed nested navigation or non-cancelable detach, the outgoing document needs one ordered lifecycle path: pagehide, transition to hidden with visibilitychange, then unload, before task/resource retirement and generation replacement. Descendants must unload before owners, handlers must see the outgoing document and stable WindowProxy, and reentrant same-frame work must not admit a new generation or resource.

Implemented boundary

• Consolidate outgoing lifecycle into a guarded pagehide → hidden visibilitychange → unload sequence.
• Apply it to allowed navigation and non-cancelable detach/discard.
• Unload detached descendants before their owners.
• Retain the outgoing Document/WindowProxy through handlers.
• Block same-frame reentrant generation/resource admission and restore the initiating src.
• Cancel outgoing-context tasks before generation advance.
• Bound hidden-root and one-shot state through activation and teardown.
• Add WPT-shaped/native source contracts and a validation/deferred-work note.

Deferred to #267/#268

Explicit reload/history lifecycle, cross-origin WindowProxy behavior, and cumulative unchanged Markdown/notebook/custom-editor/extension webview acceptance.

Validation boundary

git diff --check passed. Authored contracts were not executed under the current implementation-throughput direction.