Fix: add mutex to GlobalHistogramBinarizer for concurrent access safety

[bashhack]

Summary

• Add sync.Mutex to GlobalHistogramBinarizer to protect GetBlackRow() and GetBlackMatrix() from concurrent access
• Both methods mutate shared luminances and buckets fields via initArrays() with no synchronization, chains data races under concurrent use

Context

This complements PR #75 (sync.Once on GetBlackMatrix) and PR #81 (BitMatrix.Clone in parser). Together they address the three concurrent access points in the decode pipeline.

Testing

• All existing test packages pass, apart from an upstream, unrelated failure in TestDefaultGridSampler_SampleGrid of course