Add iOS 27 Swift Charts API support

[andywkff]

Scope

Audit the latest Xcode 27 beta Swift Charts API surface, compare it with SimpleChart, and add package support for confirmed iOS 27 additions.

Acceptance criteria

• Confirm new APIs from current Apple documentation and Xcode 27 SDK metadata.
• Add only missing package support with native availability/compiler guards.
• Add focused regression coverage and update public support documentation.
• Pass focused tests, full swift test, warnings-as-errors build, and diff checks.

[andywkff]

Audit checkpoint: Xcode 27 beta 5 (27A5237l) adds unified SwiftUI ViewBuilder composition for Chart/Plot and iOS 27 ChartContent conformances for empty, conditional, and variadic tuple content. The SDK diff found no new mark, plot, scale, or chart types. Added SCNativeChart with Swift 6.4 ViewBuilder support and a pre-6.4 ChartContentBuilder fallback; focused tests and warnings-as-errors library builds pass on both Xcode 26.6 and Xcode 27 beta 5.

[andywkff]

Completed. Added SCNativeChart with Swift 6.4/Xcode 27 ViewBuilder composition and a ChartContentBuilder fallback for older compilers, plus focused coverage and public docs. Verification: warnings-as-errors library builds passed on Xcode 26.6 and Xcode 27 beta 5; focused conditional-content tests passed on both; full suites passed 78/78 on both; Xcode 27 beta 5 DocC build passed; git diff --check passed. Audit scratch data was removed.

[andywkff]

Release follow-up completed in ab688ea4ccdc9c7e37ae166383b743d2c2406712.

• Added README and composed-chart tutorial examples for SCNativeChart conditional first-party content.
• Xcode 26.6: 78 tests passed.
• Xcode 27 beta 5: 78 tests passed.
• Xcode 27 beta 5 DocC build succeeded.
• Feature branch and main were pushed and now converge at the same commit.