docs: refine project description

[2dubu]

Summary

• Lead the README with PaletteKit's user-facing outcome: dominant-color extraction and adaptive SwiftUI/UIKit graphics.
• Condense the supporting capability summary to semantic swatches, static/mesh/animated graphics, and the async Sendable API.

Background

The existing introduction leads with PaletteKit's color-thief inspiration and then compresses several technical attributes into a dense paragraph. That framing makes the project's distinct value harder to scan and no longer matches the clearer value proposition used in the repository About description.

Solution

Replace the introductory paragraph with a concise outcome-first description, followed by one short capability summary. Attribution to color-thief remains in the dedicated Acknowledgements section.

Related Issue

N/A

Verification

• Tests added or updated: None; this is a README-only copy change with no behavioral impact.
• Commands run: git diff --check, swift test
• Environment and platforms: macOS 26.6.2 (arm64), Swift 6.3.2; 3 XCTest tests and 59 Swift Testing tests passed, with 1 availability-gated live test skipped.

Compatibility and Impact

• Public API: None
• Behavioral impact: None
• Performance impact: None
• Affected platforms: None; documentation only

Additional Context

The repository About description was updated separately to use the same outcome-first sentence.

Checklist

• This pull request is focused and contains no unrelated changes.
• Tests were added or updated where behavior changed, or I explained why they are not needed.
• Regression tests cover the intended behavior or root cause rather than only a reported sample.
• Public API changes are documented and reflected in the changelog, or this pull request has none.
• I considered source compatibility, behavioral changes, and performance impact.