[shadcn] Project Base Scroll Area into the shadcn design language

Classification

• Ledger: shadcn design-language projection
• Base owners: P-BASE-SCROLL-AREA, P-BASE-SCROLL-AREA-VIEWPORT, P-BASE-SCROLL-AREA-SCROLLBAR, P-BASE-SCROLL-AREA-THUMB (status: draft)
• New Base protocol: none

Goal

Project the current merged host-mediated Base Scroll Area family into @proto.ui/prototypes-shadcn without creating a second scrolling or movement engine.

Contract boundary

• Base owns the logical Scroll surface, Viewport attachment, Scroll/Move facts and requests, system/composed projection choice, and bounded control leases.
• The shadcn projection owns only visible Root/Viewport/Scrollbar/Thumb structure and design tokens.
• The frozen 2026-08-13 shadcn component is comparison evidence; Web DOM scrollbar details do not become portable guarantees.

Delivery surface

• packages/prototypes/shadcn/src/scroll-area/
• P-SHADCN-SCROLL-AREA, P-SHADCN-SCROLL-AREA-VIEWPORT, P-SHADCN-SCROLL-AREA-SCROLLBAR, and P-SHADCN-SCROLL-AREA-THUMB
• exact package/root exports and public CLI registry projection
• packages/prototypes/shadcn/test/scroll-area.test.ts
• EN/ZH docs at apps/www/src/content/docs/{en,zh-cn}/ui-libraries/shadcn/scroll-area.mdx
• real demo apps/www/src/content/docs/zh-cn/demo-shadcn-scroll-area.demo.ts
• preview registry, component preset/token closure, and generated Agent projection

Acceptance criteria

• Root, Viewport, Scrollbar, and Thumb consume the Base family without duplicating Scroll or Move state.
• Both system and composed projection boundaries remain valid.
• Directional track/thumb geometry, disabled/passive behavior, and teardown have executable evidence.
• Exact exports and absence of host-specific public scrollbar APIs are asserted.
• Docs distinguish portable Base facts from shadcn/Web presentation.
• The real package export renders through Web Component, React, and Vue previews.
• Focused tests, catalog, preset/token, types, docs, and proportional workspace checks pass.

Out of scope

• A new scroll engine, virtualizer, carousel, or list protocol.
• Changes to Scroll, Move Gesture, or host-capability contracts.
• Treating native CSS/DOM scrollbar APIs as cross-host props.