Stop allocating the BJData excluded-marker list per container

[nlohmann]

What & why

write_ubjson() built a std::vector of the eight markers BJData forbids as the type of an optimized container ([, {, S, H, T, F, N, Z). It did so for every array and object it wrote with use_type, one heap allocation and a linear search each, even for plain UBJSON output, which never consults the list. The literal was also spelled out twice, kept in sync only by convention.

The change

A static constexpr helper, is_bjdata_excluded_type_marker(), replaces both vectors and std::find calls. It is the same set the reader rejects (bjd_optimized_type_markers), and nothing else changes.

Verification

• Byte-identical: a differential against develop over 28,400 outputs (22,720 of them UBJSON/BJData) is identical in C++11 and C++20. It covers json and ordered_json, every use_size/use_type combination, BJData drafts 2 and 3, and the vector and adapter overloads. A mutant that drops S and Z from the helper changes 2,203 of those outputs, so the differential does exercise it.

• unit-ubjson and unit-bjdata pass. They build cleanly with CI's GCC warning flags in C++11 and C++26, and clang-tidy reports no new findings.

• Speed (clang -O3, median of 41 runs, 20,000 small typed containers):

  workload	develop	this PR
  to_ubjson(j, true, true), arrays and objects of numbers	1.71 ms	1.15 ms
  to_bjdata(j, true, true), arrays and objects of numbers	1.76 ms	1.16 ms
  to_ubjson(j, true, true), arrays of strings	1.42 ms	0.91 ms
  to_bjdata(j, true, true), arrays of strings	1.52 ms	0.89 ms

#5518 moves both call sites into new functions; whichever of the two merges second needs a two-line adjustment.

Public API impact

No breaking changes. Same output, and only a private helper was added.

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• The changes are described in detail, both the what and why.
• If applicable, an existing issue is referenced. (none)
• The Code coverage remained at 100%. A test case for every new line of code.
• If applicable, the documentation is updated. (not applicable)
• The source code is amalgamated by running make amalgamate.

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