Add C header nullability annotations

[Jesssullivan]

Goal

Improve Swift and Objective-C diagnostics for include/zig_crypto.h without changing the stable C ABI.

Context

The current header is plain C and works for C consumers. Apple interop would benefit from carefully scoped nullability annotations so Swift/ObjC users get clearer import behavior and warnings.

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

Scope

• Audit pointer parameters in include/zig_crypto.h for nullability annotations that do not change ABI.
• Add annotations using standard Objective-C/Clang-compatible macros or guarded definitions.
• Preserve C compatibility for non-Apple compilers.
• Update docs if the import behavior changes for Swift/ObjC users.

Acceptance

• Header remains usable from C and Zig-built examples.
• Existing zig build test, zig build test-pbt, and zig build example still pass.
• The change does not add ObjC classes, Swift wrappers, or new crypto behavior.