Safe Strings & OwnedString Interop

[AKRiLLiCK]

• Description: Implement the immutable String slice wrapper and the OwnedString type for allocator-aware formatting.
• Acceptance Criteria:
  • Implement STRING_new.
  • Implement STRING_format and STRING_formatVariadic utilizing snprintf/memccpy safely under the hood.
  • Tests must verify that STRING_format routes through the provided Allocator, returns a valid OwnedString via the Monad union, and properly handles allocation failures.