Prove and promote atomic renderer slots and mixed-renderer islands

[woksin]

Problem

Components 4 stabilizes only the nine element-bounded stable-presentation/v1 slots. Tooltip, dropdown, dialog, date picker, and paginator remain behind the generic unstable_ renderer surface because a real vendor has not yet proved complete atomic interaction ownership.

Promoting those slots without proof could layer two focus traps, dismissal systems, scroll locks, keyboard models, or ARIA owners in one interaction.

Direction

Satisfy proof gate G7 with one real vendor-owned interaction, preferably dialog:

• adapter declares mode: 'atomic';
• Core emits no competing behavior for that slot;
• exactly one focus trap, scroll lock, dismissal owner, focus-restoration path, and ARIA state owner exists;
• portal container, theme inheritance, z-index, SSR, hydration, reduced motion, RTL, and forced colors are exercised;
• then prove a real two-renderer RendererScope island with deterministic fallback and remount behavior.

Keep the stable presentation profile unchanged. Interaction promotion should use a separately versioned profile rather than silently adding requirements to v1.

Acceptance criteria

• A concrete vendor adapter passes full behavior-ownership conformance for one atomic slot.
• Planted double-owner defects fail deterministically.
• A two-renderer island passes nested overlay/focus/portal tests.
• Public semantics, failure diagnostics, and remount/state-loss behavior are documented.
• Only the proven atomic contract is promoted; unsupported interaction slots remain unstable.

Non-goals

• No transparent full-catalog replacement.
• No vendor DataPage/DataTable claim.
• No weakening of Cratis-owned composite or Arc orchestration contracts.