Add Button support

[Kyle-Ye]

Summary

Complete public Button support so buttons render their labels and invoke their actions through supported input mechanisms.

Scope

• Complete the generic Button implementation and its common label, title, and role initializers.
• Connect pointer or touch activation to the button action, including disabled-state handling.
• Integrate ButtonStyle, PrimitiveButtonStyle, their configuration values, .buttonStyle(_:), and baseline default, plain, and borderless styles.
• Propagate label, role, enabled, and pressed state through the style pipeline.
• Add examples and focused coverage for rendering, activation, disabled buttons, roles, and custom styles.

Acceptance criteria

• Button("Run") { ... } renders its label and invokes its action once per activation.
• A disabled button does not invoke its action.
• Custom button styles receive the expected label, role, and pressed state.
• Primitive button styles can trigger the configured action.
• Button activation participates in the supported host's normal focus and accessibility behavior.

API reference

• Button