Components: Provide modal focus scope and background accessibility isolation #4379

Goal

Establish the native modal foundation needed by Dialog #4222, Drawer #4223, and modal Popover policy in #4236. Part of #4274. Existing component-generation issues remain the delivery owners; this issue owns their shared native prerequisite.

Acceptance

• Review host/window ownership, arbitrary-child focus containment, initial focus, Escape/close, nested modality, and safe restoration as a modal contract distinct from FocusZone navigation.
• Implement actual background accessibility isolation and scrim/input containment with attachment/window/generation-scoped ownership and idempotent teardown.
• Test forward/backward Tab, programmatic and assistive-technology focus escape, nested popup ownership, owner loss/unmount, close/reopen races, and restoring only when policy still permits it.
• Qualify native keyboard and assistive-technology behavior across the declared macOS Paper/Fabric, Windows, and Win32 capability matrix; record refusals/unsupported paths explicitly.
• Keep root/all-host admission gated by equivalent mandatory semantics; cycling a FocusZone or showing an anchored Callout is not modal qualification.

Evidence

Retired handoff: b55d6c9a3:packages/agentic/native-core/plans/modernization.md. Durable boundaries: native-core README/AGENTS and modern Callout/FocusZone SPEC.md files. Reuse proven host services without introducing a global view-tag registry or speculative cross-platform framework.