Chroma integration: shared color roles across NMSh UI

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Follow-up to the exploratory visual-effects/color-treatment architecture in #78; this issue covers implementation integration that #78 intentionally leaves undecided.

Goal

Route color choices for NMSh-owned components through shared Chroma primitives rather than local color/gradient calculations.

Scope

• Solid colors, semantic roles and theme colors.
• Linear and multi-stop gradients usable by text, frames and fills.
• Language identity colors as a distinct category (see Standardized language identity colors from GitHub Linguist #176; language color must never imply success/warning/failure).
• Luminance curves used by motion and capability-aware fallback for truecolor, 256-color and no-color terminals.
• A clear boundary: Chroma determines which colors/effects cells receive; UI components determine where cells exist.

Preserve raw PTY output and /copy unchanged. This is not a layout engine and must not duplicate #78's research scope.

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Shipped in v0.7.0.