Fix: bitmatrix clone for concurrent decode

[bashhack]

Hey @makiuchi-d - hope you're doing okay, apologies for the long gap between contributions - but I have one more to propose.

Summary

BitMatrixParser mutates its input BitMatrix during decoding (UnmaskBitMatrix, Remask, Mirror), but since #75 introduced sync.Once caching in BinaryBitmap.GetBlackMatrix(), all concurrent decoders share the same cached matrix instance. When multiple goroutines decode from the same BinaryBitmap, one goroutine's Flip() calls corrupt the matrix while another is reading, causing occasional NotFoundException with invalid coordinates (e.g. (53, -2)).

This then is a follow-up to my previous contribution #75, which fixed the GetBlackMatrix() race but exposed this deeper mutation-sharing issue.

Changes

• BitMatrix.Clone(): Implements the deep copy method (stub comment already existed: // public BitMatrix clone()). Returns a new BitMatrix with an independent copy of the backing []uint32 slice.
• NewBitMatrixParser: Clones the input matrix so that decode mutations (UnmaskBitMatrix, Remask, Mirror) operate on a private copy rather than the shared cached original.

Why clone in the parser, not elsewhere?

• Removing sync.Once caching from GetBlackMatrix() would work but forces expensive re-binarization on every call. Cloning a BitMatrix is a single copy() of the backing slice.
• The decoder is the component that needs private mutable state — I still feel the cache is correct to share read-only data.
• DataMatrix's BitMatrixParser is unaffected: it extracts a data region into a new matrix and never mutates the input.

Behavioral change

Previously, Decoder.Decode(bits, hints) mutated bits as a side effect (left it in an unmasked/mirrored state). Now bits is untouched after decode. This is arguably more correct, I think, as the mutations are implementation details of the decode retry logic, not a documented contract. No code in the repository reads the BitMatrix after decoding.

Testing

Updates and added tests, of course - but while I was here, I also tackled that TestDefaultGridSampler_SampleGrid failure.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.93%. Comparing base (95e256b) to head (e2a5d3b).

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #81   +/-   ##
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  Coverage   97.93%   97.93%           
=======================================
  Files         122      122           
  Lines       12951    12955    +4     
=======================================
+ Hits        12684    12688    +4     
  Misses        191      191           
  Partials       76       76           

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