Split unit-regression2.cpp so the MinGW linker can relocate it

[nlohmann]

Prerequisite for the binary-reader series (#5104, PRs #5501–#5508); it does not change library code.

What

Linking test-regression2 with clang and MinGW fails once the translation unit grows past a certain size:

relocation truncated to fit: IMAGE_REL_AMD64_REL32 against `.rdata'

The code can no longer reach the read-only data it references within the range of a 32-bit relocation. unit-regression2.cpp is one of the largest files in the test suite and had been sitting just under that limit, so a change anywhere in the library is enough to tip it over — the iterative binary readers in this series do exactly that, on 9 of 9 clang versions.

Two things worth noting:

• This is the second time this file has hit a size limit: it exists because unit-regression1.cpp was split for size.
• windows.yml already carries a workaround for the same limit hitting the same target — -DCMAKE_CXX_FLAGS_DEBUG="-g0", because linking it failed with IMAGE_REL_AMD64_SECREL against '.debug_line'. Dropping debug info fixed that relocation. This one is against .rdata, which no compiler flag avoids.

How

Move the second half of the regression tests, and the helper types only they use, into unit-regression3.cpp. The sections are independent — every statement in TEST_CASE("regression tests 2") was already inside a SECTION — so they move unchanged.

Nothing is lost: 168 assertions before the split, 50 + 118 after.

Both files end up comfortably smaller than the one that used to link (clang, -O1, C++20):

	read-only data	text	object
before	58,233	1,287,764	3,158,120
unit-regression2.cpp	48,161	1,012,988	2,522,296
unit-regression3.cpp	41,710	772,704	1,878,880

No CMake change is needed: tests/CMakeLists.txt globs src/unit-*.cpp, so the new file is built for every standard like its siblings.

CONTRIBUTING.md pointed contributors at unit-regression2.cpp for new bug tests; it now points at the smaller file and explains why the two exist, so the split doesn't quietly undo itself.

API impact

No breaking changes. Test-only change; no library code is touched.

Checklist

• The changes are described in detail, both the what and why.
• If applicable, an existing issue is referenced.
• The Code coverage remained at 100%. A test case for every new line of code.
• If applicable, the documentation is updated.
• The source code is amalgamated by running make amalgamate.

🤖 Generated with Claude Code

[gregmarr]

"growing the larger one"?

[nlohmann]

Will fix - once the running CI jobs finish.