[shadcn] Project Base Textarea into the shadcn design language

Classification

• Ledger: shadcn design-language projection
• Base owner: P-BASE-TEXTAREA (status: draft)
• New Base protocol: none

Goal

Project the current merged host-mediated Base Textarea protocol into @proto.ui/prototypes-shadcn while keeping the physical plain-text multiline editor and editing lifecycle host-owned.

Contract boundary

• Base owns controlled/uncontrolled value, normalized input/change/IME events, accessibility projection, focus/blur, and one physical editor.
• The shadcn projection owns classes, sizing, border, focus-ring, disabled, and invalid presentation only where those states already exist.
• The renderer stays contentless; authored children do not become a fake editor.

Delivery surface

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

Acceptance criteria

• The projection consumes Base Textarea and materializes exactly one host-owned multiline editor.
• Controlled/uncontrolled value, dirty-value preservation, IME, live properties, accessibility, and physical focus behavior remain unchanged.
• No second editor, authored-child renderer, form workflow, validation message, auto-resize, rich-text, or announcement protocol is introduced.
• Exact exports and supported visual states are asserted.
• Docs distinguish Base editing guarantees from shadcn visual choices.
• 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

• Base Input or a generalized Native Control protocol.
• Auto-grow, rich text, form submission, and validation orchestration.