Fix const iterator value types

[fzlzjerry]

Description

Make the iterator value_type cv-unqualified so generic code using std::iterator_traits<It>::value_type can collect const YAML ranges into a std::vector.

Keep the existing cv-qualified dereference result for both forward and reverse iterators. Pointer/reference types, iterator categories and Node sharing behavior are unchanged. Changing the value-type alias alone would also remove const from both dereference results; the regression tests cover that distinction.

Validation

• Five regression tests cover traits/constness, sequence and map collection in both directions, empty ranges and Node identity. They fail to compile on the original base because of std::vector<const iterator_value>.
• Full CMake/CTest runs pass with GCC 14.2 (C++11-configured Debug and C++20 Release) and Clang 19.1.7 (C++23 shared library with ASan, UBSan and leak detection). The 8 existing disabled tests are unchanged.
• Standalone consumers compile and run with both compilers in C++11, C++20 and C++23; separate installed-package checks pass for all four iterator modes against each build. The C++11 CMake profile uses C++11 for the library and C++14 for GoogleTest; the standalone C++11 checks use C++11 throughout.
• Changed-code clang-format, whitespace checks and git diff --check pass.

Closes #1326.

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