Document the component contribution contract #2

Give contributors a concrete, consistent definition of a completed Gea UI contribution.

Part of Gea UI Elements — Community Contributions.

Area: Shared contribution infrastructure

Scope and acceptance criteria

• Document API/event naming, state ownership, cleanup, package placement, and typed native representations.
• Require reuse of existing primitives and link existing implementations before proposing new ones.
• Define PR expectations: usage example, behavior tests, target evidence, documentation, and explicit limitations.

Existing implementation to reuse

• core/README.md
• core/packages/elements/README.md
• core/packages/core/index.d.ts

ScrollView, Canvas, Image/GIF, CameraView, VirtualList, virtual keyboard, Label/Text, TextField, TextArea, and Button already exist. Reuse those facilities; this issue does not request rebuilding them.

Validation

• Demonstrate the guidance or tooling against existing components.
• Record tested target behavior separately from source-only findings and unsupported paths.