Fix ADL leak of nlohmann::detail through basic_json's default base class

[nlohmann]

Summary

Fixes #4320.

Since #3110 (3.11.3), basic_json unconditionally derives from detail::json_base_class<CustomBaseClass>, which resolves to detail::json_default_base when no custom base class is supplied (the default, CustomBaseClass = void). Because json_default_base lived in namespace nlohmann::detail, every basic_json specialization gained nlohmann::detail as an associated namespace for argument-dependent lookup (ADL) — even for users who never touch the custom-base-class feature.

This silently exposes the library's internal, ADL-only generic to_json overloads (e.g. the compatible-array-type overload in detail/conversions/to_json.hpp) to any unqualified to_json()/from_json() call a user makes involving a basic_json argument.

In #4320, the user's to_json for one type delegates via an unqualified to_json() call to a base type's to_json — a common pattern for thin wrapper/adapter types (there, a class deriving from Eigen::Vector3d). Because the delegated-to argument's type also has begin()/end() (true of Eigen::Vector3d since Eigen 3.4), overload resolution now also considers the library's own generic array serializer, newly visible via the nlohmann::detail ADL leak. That overload is an exact-match template for the derived type, whereas the user's intended overload requires a derived-to-base conversion — and an exact match beats a non-exact conversion regardless of template-ness, so the user's serializer is silently bypassed and the object gets serialized as [x,y,z] instead of {"x":x,"y":y,"z":z}.

Fix

Move json_default_base out of nlohmann::detail into nlohmann itself. detail::json_base_class<T> keeps its alias-template location — alias templates don't affect ADL, only the referenced type's namespace does — so this doesn't change basic_json's effective base class, json_base_class_t's public signature, or any other public API. It only removes the accidental ADL exposure of nlohmann::detail for the default (no custom base class) case.

Public API is unchanged

Two distinct names are involved here, and it's worth being precise about which one actually moved:

• detail::json_base_class<T> — the alias template that implements the void → default switch, and that json_base_class_t is documented in terms of (using json_base_class_t = detail::json_base_class<CustomBaseClass>;). This stays in nlohmann::detail, unchanged.
• json_default_base — the empty placeholder struct that json_base_class<T> resolves to when T = void. This is the one that moves, from nlohmann::detail::json_default_base to nlohmann::json_default_base.

json_default_base was never part of the documented API — only CustomBaseClass and json_base_class_t are documented, and neither of those names, namespaces, or signatures change. json_base_class_t's definition still resolves through the same nlohmann::detail::json_base_class alias template it always did; only the value that alias produces for the default (no custom base class) case moves namespace. So no documented name, header path, or template signature changes — the only observable effect is the ADL fix itself.

Test plan

• Added a regression test in unit-regression2.cpp reproducing the exact overload-resolution scenario from Behavioral change of serializers in 3.11.3. Name lookup related. #4320 (derived-to-base delegation to a begin()/end()-bearing base type's to_json), using a minimal stand-in for the Eigen type involved (no Eigen dependency needed).
• Verified the new test fails without this fix ([1.0,2.0,3.0] instead of {"x":1.0,"y":2.0,"z":3.0}) and passes with it.
• tests/src/unit-custom-base-class.cpp, unit-regression2.cpp, unit-udt.cpp, unit-udt_macro.cpp all pass against both the modular headers and the amalgamated single_include header, at C++11/14/17/20, with -Wall -Wextra -Werror.
• Ran make amalgamate (astyle 3.4.13) — single_include/nlohmann/json.hpp is up to date.

(This PR was prepared by Claude Code on behalf of @nlohmann, following up on the maintainer's own root-cause investigation of #4320.)