macOS: add traffic‑light inset API and demo example

[AndrewJSchoen]

• Tested on all platforms changed
• Added an entry to the changelog module if knowledge of this change could be valuable to users
• Updated documentation to reflect any user-facing changes, including notes of platform-specific behavior
• Created or updated an example program if it would help users understand this functionality

Description:

• Adds WindowAttributesMacOS::with_traffic_light_inset and WindowExtMacOS::set_traffic_light_inset to offset the traffic‑light buttons in logical points.
• Applies the inset reliably across title changes, resizes, and fullscreen transitions; no effect when titlebar buttons are hidden.
• Adds traffic_lights example for interactive adjustment (arrow keys, Shift for coarse, R reset, Esc exit).
• Adds changelog entry in winit/src/changelog/unreleased.md.

Fixes #4436

Tests:

• cargo check -p winit-appkit
• cargo check -p winit --example traffic_lights

[AndrewJSchoen]

I think this should be ready to go if everything checks out with the owners.