Add std::span packing support and extend span tests

[bacek]

• Add pack_type overload for std::span<B const, E> that serialises as msgpack bin8/bin16/bin32
• Add packSpanRoundTrip: pack span → unpack vector, compare bytes
• Add packSpanMatchesVector: packed bytes from span == from equivalent vector

[codecov]

Codecov Report

❌ Patch coverage is 91.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 93.85%. Comparing base (6639136) to head (55c7159).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
include/msgpack23/msgpack23.h	91.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main       #8      +/-   ##
==========================================
- Coverage   93.90%   93.85%   -0.06%     
==========================================
  Files           1        1              
  Lines         476      488      +12     
==========================================
+ Hits          447      458      +11     
- Misses         29       30       +1     

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