Model nested reload and history traversal #585

Parent: #267. Webview epic: #264.

Problem

Nested documents need bounded per-frame history and explicit reload/traversal lifecycle. history.back()/forward()/go(), same-document entries and location.reload() must preserve stable WindowProxy identity, order popstate/hashchange, retire outgoing work for cross-document reload, and reject reentrant lifecycle corruption.

Implemented boundary

• Add bounded per-frame same-document history state/index.
• Queue back, forward and go traversal.
• Dispatch popstate then hashchange where applicable.
• Implement nested location.reload() through the established beforeunload/pagehide/visibility/unload path.
• Preserve stable WindowProxy/history identity across same-document traversal and reload replacement.
• Guard lifecycle reentrancy and retire outgoing tasks, fetches, listeners and ServiceWorker ownership.
• Add WPT-shaped/native source contracts and a validation/deferred-work note.

Deferred to #267/#268

Cross-document traversal, joint session history, POST reload, bfcache, restricted cross-origin WindowProxy behavior, and cumulative unchanged webview acceptance.

Validation boundary

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