Tint: accent color for controls

[colemancda]

.tint(_:) sets the accent color for controls in a subtree — an environment value, like .foregroundColor.

What's new

• .tint(_ color:) modifier (emits kind tint).
• A LocalTint CompositionLocal folded in RenderChild alongside font/color/disabled, so it propagates to the whole subtree.
• Applied to the controls that honor it: Button (container), Toggle (Switch track), Slider (thumb + active track), ProgressView (indicator).

Verification

• swift test — new emission test, 65 total passing
• Emulator (Styling screen, new "Inherited tint" example): a .tint(.pink) container turns its button, toggle, slider, and progress bar all pink; the other inheritance examples are unaffected

Deferred

Tint on Picker/Stepper/DatePicker accent (uses default) and Menu items.