fix: emit afterClosed only after the native modal dismissal completes - #179

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What is the current behavior?

NativeDialogRef.afterClosed() is unreliable: it is usually emitted by a hardcoded 100ms fallback timer instead of the actual dismissal. On iOS the modal dismiss animation takes ~400–500ms, so afterClosed fires while the previous view controller is still being dismissed — opening another dialog from afterClosed then fails with Failed to open dialog: the modal view could not be presented, forcing apps to work around it with timeouts.

Additionally:

• afterOpened() never fires (nothing ever emitted the 'opened' state).
• Template-based dialogs dismissed natively (back button / swipe-down) never emit afterClosed at all.
• The template-dialog close path decrements the modal navigation depth twice.

What is the new behavior?

afterClosed is now driven by core's showModal closeCallback, which fires only once the native dismissal has completed (on iOS, from the dismissViewControllerAnimated completion handler), followed by a one-shot unloaded listener attached only at that point — so an unrelated unload (e.g. the app going to the background) can never masquerade as a close. The old timer remains only as a 5s safety net for dismissals that never report completion.

• Opening a new dialog from afterClosed now works without workarounds.
• afterOpened() fires once the modal is fully presented (wired to shownModally).
• Component and template portals share the same show/dismiss path, so natively-dismissed template dialogs now emit afterClosed, and _closeModalNavigation runs exactly once per close.
• New native-dialog specs (passing on iOS and Android) assert the behavior: beforeClosed→afterClosed ordering and result value, no ancestor still presenting at afterClosed time, an immediate no-timeout reopen, and native dismissal of both dialog types.

Note: afterClosed now fires at the real dismissal completion (~400–500ms on iOS) instead of ~100ms — intentionally later, since the earlier timing was a lie.