cstrings: add NSStringToString

[TotallyGamerJet]

What issue is this addressing?

Closes #438

What type of issue is this addressing?

feature

What this PR does | solves

Add function NSStringToString to convert an NSString ID to a Go string.

I put this in a new package cstrings so as to not create a dependency on Foundation framework in the objc package. This package would also be a good spot to export the internal strings.GoString function.

[hajimehoshi]

I put this in a new package cstrings so as to not create a dependency on Foundation framework in the objc package.

I understand why we need a separate package. Do we have a plan to put other things than objc helpers?

If we need helpers for the objc package, what about objcutil?

[hajimehoshi]

Another safer bet is puregoutil.

[Copilot]

Pull request overview

Adds a Darwin-only helper to convert Objective-C NSString objects (passed as objc.ID) into Go string, addressing the common footgun around UTF-8 vs UTF-16 length handling (Issue #438).

Changes:

• Introduces new cstrings package with NSStringToString(objc.ID) string on Darwin.
• Loads Foundation at init-time to ensure NSString/related classes are available.
• Adds Darwin-only tests for nil input, empty string, normal string, and non-NSString panic behavior.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
cstrings/nsstrings_darwin.go	Adds NSStringToString implementation and init-time Foundation/selector/class setup.
cstrings/nsstrings_darwin_test.go	Adds unit tests validating conversion behavior and panic on non-NSString input.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[hajimehoshi]

LGTM, thanks!

[hajimehoshi]

Still LGTM