webview: render GPUI overlays above native WebView - #2626

Summary

• enable GPUI's layered macOS scene when creating a wry WebView
• render Dialog, Sheet, and Notification root layers in the deferred overlay plane
• expand the WebView example to cover Popover, Dialog, Notification, and PopupMenu in an 800x600 window
• keep native WebView focus ownership in gpui-wry, including stale-caret cleanup
• install standard Edit-menu actions so AppKit routes clipboard shortcuts through the active responder
• add an English platform-composition report and Windows CompositionController implementation handoff
• depend directly on the companion GPUI branch in Cargo.toml

Depends on zed-industries/zed#61945

Current scope

The macOS visual stack, focus handoff, clipboard shortcuts, and modal/outside-click behavior are validated. While any top-plane primitive exists, the full transparent overlay view captures input; fine-grained pass-through regions remain part of the documented production native-surface portal design. Windows CompositionController and Linux hosting are documented follow-ups rather than claims of this PR.

Focus and clipboard ownership

gpui-wry owns the native focus boundary instead of adding WebView-specific behavior to GPUI. A lifecycle-scoped local NSEvent monitor native-hit-tests direct WKWebView clicks and clears GPUI logical focus. The existing GPUI mouse listener covers the overlay-active path, including dismissing a Popover by clicking the WebView region.

The example installs standard Edit-menu actions for Cut, Copy, Paste, and Select All. AppKit routes Cmd+C, Cmd+V, Cmd+X, and Cmd+A through the active WKWebView or GPUI responder.

Test Plan

• cargo check -p gpui-wry -p webview
• manually verify Popover, Dialog, Notification, and PopupMenu render above a live WKWebView
• manually verify clicking WebView dismisses Popover
• manually verify a direct WebView click clears the address Input focus indication and stale caret
• manually copy from GPUI to WebView and from WebView to GPUI using Cmd+C and Cmd+V
• git diff --check