Select all text in active nested webview documents #440

Parent: #268. Webview epic: #264.

Proven gap

Unchanged Code OSS sends its Select All command to the active webview as target.contentDocument.execCommand('selectAll'). Current WebScene implements the legacy clipboard commands but returns false for selectAll, even though the nested realm already has live Selection state from #405.

Focused scope

• Implement case-insensitive selectAll for the current HTML document realm.
• Reuse the realm-local Selection object and keep the range inside the active nested document.
• Preserve replacement/detach cleanup and opaque/cross-origin denial already owned by Enforce secure nested-document lifecycle for Code OSS webviews #267.
• Keep copy/cut/paste authority under Add a bounded async Clipboard API host ABI for native runtimes #86; do not add host requests or editing mutation.

Deferred validation debt

The implementation is being merged without new WPT, Chromium-oracle, native regression, performance, memory, or lifecycle runs under the current implementation-only directive. Follow-up acceptance must cover nested Markdown documents, multi-node text serialization, empty documents, replacement/detach reset, 100-cycle bounds, and the exact unchanged Code OSS Select All command path.

Non-overlap

Do not touch consolidation PR #76, AppScene, vscode-demo, CSS, File APIs, accessibility publication, or clipboard authority.