feat(Modal): add nesting behaviors and fix third-party overlay dismissal

[stephenjwatkins]

📝 Changes

Adds control over how nested modals stack, and fixes a dismissal bug for modals that host third-party overlays.

Nesting behaviors (feature). New optional childNestingBehavior and selfNestingBehavior props on Modal.Trigger and ModalContainer control how a modal stacks relative to the modal it's nested under. A connection resolves to one of:

• stack (default) — both modals keep their own backdrops; matches today's behavior, so existing modals are unchanged.
• stack-shared-backdrop — the nested modal suppresses its backdrop so only the lowest backdrop shows.
• replace — the nested modal hides the modal beneath it so only the topmost is visible.

Configure from either end: childNestingBehavior is set on the parent and cascades down the tree; selfNestingBehavior is set on the child, is local (does not cascade), and overrides the parent's childNestingBehavior for that one connection — useful for surgically adjusting a single nested modal without touching its parent.

Third-party overlay dismissal (fix). Previously the modal box was tagged data-react-aria-top-layer, which made react-aria treat every click inside the modal as "not outside," so nested react-aria overlays (e.g. Select, Menu) wouldn't close on outside click. The box now uses data-live-announcer instead, which ariaHideOutside honors to keep the box visible under a surrounding modal without breaking outside-click dismissal. A new global shouldCloseOnInteractOutside also ensures clicking a nested modal never dismisses the one beneath it, while backdrop clicks and genuine third-party overlays (e.g. Stripe) still behave correctly.

Notes for reviewers

• No breaking changes. The new props are optional and default to today's behavior. Removed context fields (hasOpenNestedModal/setHasOpenNestedModal) were internal-only and unused on main.

• New nesting logic is isolated in a dedicated useModalNesting hook; the only shared-path change (shouldCloseOnInteractOutside) is behavior-preserving for existing single modals (verified: clicking a Select option or the modal body does not dismiss a standard modal; backdrop dismissal is unchanged).

• Try the Modal → Nested story; the childNestingBehavior control switches between all three modes live.

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