Fix: Use NSRecursiveLock for ThreadSafeSingleton

[rickhohler]

Switches from NSLock to NSRecursiveLock in ThreadSafeSingleton to allow for safe nested singleton initialization (e.g. valid subclass singletons initializing other singletons in their init).

Additional Improvements (v1.1.1)

• Concurrency Fixes: Resolved Sendable conformance warnings and unnecessary await calls in MergerTests and internal classes.
• Hashing Enhancements:
  • Added convenient Data extensions: sha256 and sha256Hex.
  • Improved HashError to conform to LocalizedError for better error messages.
• Documentation Updates:
  • Updated CHANGELOG.md to version 1.1.1.
  • Updated EXAMPLES.md to show new hashing extensions.
  • Corrected ThreadSafeSingleton usage example in code comments to include required createShared() override.
• Cleanup: Removed temporary test_import.swift file.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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