Adds an NSString foreign class and impl Into(*NSString) for string so a string literal flows into any *NSString slot via xx. uikit's keyboard userInfo lookups now read objectForKey(xx "...") instead of ns_string("...".ptr), and objectForKey's key param is retyped *NSString.
ffi-objc-call-06 .ir snapshot regenerated: declaring the NSString type adds its reflection thunks (struct_to_string/pointer_to_string), same as the existing NSObject/NSDictionary. Runtime output unchanged.
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