Add README bibliography and positioning

[basiclines]

Summary

This PR documents how TUIkit relates to headless UI primitive libraries and LLM/spec-to-code research so reviewers and contributors have clearer context for the project model.

Changes

• Adds a README positioning section that frames TUIkit as a spec-first, headless primitive system for terminal UIs.
• Adds a bibliography covering Radix Primitives, Base UI, accessibility references, and LLM compiler/specification-to-code research.

Testing

Documentation-only change; not run.

Checklist

• Specs lint passes (bun run lint)
• Tests updated if applicable
• Documentation updated if applicable

[Copilot]

Pull request overview

This PR updates the project documentation to better explain TUIkit’s conceptual model (spec-first, headless primitives for terminal UI) and to ground that positioning with relevant prior art and research references.

Changes:

• Adds a “Positioning” section describing how TUIkit relates to headless UI primitive libraries and spec-first generation workflows.
• Adds a “Bibliography” section with references for headless primitives, accessibility patterns, and LLM/spec-to-code research.

Show a summary per file

File	Description
README.md	Adds a new positioning section and a bibliography to clarify project context and related work.

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• Files reviewed: 1/1 changed files
• Comments generated: 2