Add SwiftPM modulemap smoke test

[Jesssullivan]

Goal

Make the current C ABI easy to import from Swift without changing notification behavior.

Context

zig-notify exposes include/zig_notify.h and builds libzig-notify.a. The Apple interop docs identify SwiftPM/modulemap packaging as a current parity gap.

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

Scope

• Add a minimal SwiftPM/modulemap smoke test or documented package layout that imports zig_notify.h.
• Show the header search path and static-library link setup.
• Keep the current backend truth clear: macOS uses osascript, not a full UNUserNotificationCenter replacement.
• Do not change the C ABI.

Validation

• zig build -Doptimize=ReleaseFast
• zig build example
• A small Swift import/build smoke test, if the contribution adds one.