Audit SwiftUI state privacy for lazy @State

[andywkff]

Audit SimpleChart against Point-Free's SwiftUI @State macro guidance: https://x.com/pointfreeco/status/2093052971890229285

Acceptance criteria:

• Genuine mutable @State storage is private.
• Immutable parent inputs are not modeled as @State.
• Stable and Xcode 27 beta builds/tests pass.
• No unrelated files are changed.

[andywkff]

Audit complete: 26 @State declarations were found. The 23 non-private declarations were immutable legacy config, data, and size inputs, so they have been changed to private let. The three genuine mutable state properties were already @State private. Static scan now finds no non-private @State declarations.

[andywkff]

Verification complete:

• Stable Xcode warnings-as-errors library build passed.
• Xcode 27 beta warnings-as-errors library build passed.
• Stable Xcode full suite: 78 tests passed, 0 failures.
• Xcode 27 beta full suite: 78 tests passed, 0 failures.
• Static scan finds exactly three remaining @State declarations, all private.
• git diff --check passes.

[andywkff]

Published in 12508bebeae88d5e9a89ff0dfe46b367de10b1d2 (Privatize SwiftUI state storage). The feature branch and main were fast-forwarded and pushed; local and remote refs all converge at this commit, with both worktrees clean.