Add zero-copy unpack_type overload for std::span<B const>

[bacek]

Problem

The existing binary unpack path copies data into std::vector:

value.assign(src, src + bin_size);  // always allocates + copies

For callers that only need to read the binary payload (e.g. pass it to a parser like GEOS WKBReader), this allocation is pure waste — the data is already contiguous in the Unpacker's internal buffer.

Solution

Add a second unpack_type overload that resolves when the caller declares their argument as std::span:

template<std::size_t E>
void unpack_type(std::span<B const, E> &value) {
    // ... read header to get bin_size ...
    value = data_.subspan(position_, bin_size);  // zero-copy view
    increment(bin_size);
}

data_.subspan() returns a non-owning view into the Unpacker's own buffer, so no allocation or memcpy occurs. The span remains valid for the lifetime of the Unpacker (i.e. for the duration of the unpack call chain), which is sufficient for immediate-use patterns like deserialise → parse → discard.

Safety

• Bounds are checked identically to the vector overload (position_ + bin_size

  data_.size() throws std::out_of_range).

• The returned span is const-qualified (B const), preventing accidental writes back into the source buffer.
• Callers that need to own the data should continue using std::vector.

Backwards compatibility

Fully additive — existing code using std::vector is unaffected. The new overload is only selected when the target variable is declared as std::span.

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 75.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.90%. Comparing base (b35e800) to head (a1b7c5a).

Files with missing lines	Patch %	Lines
include/msgpack23/msgpack23.h	75.00%	2 Missing ⚠️
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Additional details and impacted files

@@            Coverage Diff             @@
##             main       #7      +/-   ##
==========================================
+ Coverage   93.16%   93.90%   +0.74%     
==========================================
  Files           1        1              
  Lines         468      476       +8     
==========================================
+ Hits          436      447      +11     
+ Misses         32       29       -3     

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