fix: stabilize native iOS share handoff

[Kapusch]

What changed

• retain static Facebook Share interop and route callback URLs to ShareKit
• make managed completion idempotent when presentation failure and delegate callbacks race
• force ShareKit native-app dialog mode
• clear only Meta's private app-bridge pasteboard marker without reading the pasteboard
• document the native return behavior

Why

The automatic ShareKit mode selected a deprecated sheet path that could report premature completion. On return from Facebook, ShareKit also read the general pasteboard while cleaning its bridge payload, which triggered an iOS paste-authorization prompt.

Impact

Photo sharing opens the Facebook native editor, completes once, returns cleanly after cancellation, and no longer prompts the user to allow pasting from Facebook.

Validation

• feature-selection validation
• native wrapper build for device and both simulator architectures
• local NuGet pack and module-layout validation
• physical-device Facebook Feed handoff, cancellation callback, and return without paste prompt