Accessibility and reduced-presentation modes

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Part of #164.

Goal

Define and deliver accessibility behavior through NMSh's reusable UI primitives rather than as a separate renderer.

Investigate and specify

• Plain/screen-reader-friendly presentation where terminal interfaces allow it.
• Reduced motion and no-color/low-color fallback.
• Keyboard-only parity, visible focus and semantic labels.
• Safe glyph compatibility and readable status cues that do not rely on color alone.

Audit existing interaction surfaces and propose concrete acceptance criteria for a consistent baseline. Mouse must never be required. Coordinate reduced motion with #78 and deterministic rendering work.

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Shipped in v0.7.0.