Add Objective-C bridge sample

[Jesssullivan]

Goal

Show how an Objective-C or Cocoa-adjacent application can call the stable zig_notify.h C ABI.

Context

The repo has a C example, but no Objective-C bridge sample yet. This is a documented Apple interop gap.

Docs: https://transscendsurvival.org/zig-notify/guides/apple-interop/

Scope

• Add a small Objective-C command-line sample or documented snippet that links libzig-notify.a.
• Demonstrate zig_notify_init, zig_notify_send, and zig_notify_deinit.
• Note that the current macOS backend uses osascript and ignores urgency.
• Do not add AppKit, SwiftUI, Cocoa, or UserNotifications.framework dependencies.

Validation

• zig build -Doptimize=ReleaseFast
• zig build example
• Compile the Objective-C sample or document the exact compile command.