Add Objective-C bridge sample

[Jesssullivan]

Add a minimal Objective-C sample that calls zig_keychain_store, zig_keychain_lookup, and zig_keychain_delete through include/zig_keychain.h.\n\nScope:\n- caller-provided buffers only\n- no AppKit/UIKit/Cocoa UI\n- no iCloud Keychain, Secure Enclave, LocalAuthentication, access groups, certificates, or private-key identity claims\n- update README or docs/guides/apple-interop.md with the sample path.