[Bronek]

Restructures the compile-fail tests from #459 so they can be read top to bottom. Each scenario is a named #ifdef LIBFN_REJECT_<scenario> block in tests/pfn/reject/, right after a positive control that differs only in what makes it fail. CMake reads the guards from each source: the source must build as is, and each guard becomes a WILL_FAIL test named after it. Coverage is unchanged at 41 scenarios per mode.

Checked locally on gcc 16.2 and clang 22.1: the full suite passes, and against the pfn headers from before #459 all 41 reject tests fail while both sources still build. This PR's CI is the first MSVC run.

Assisted-by: Claude:claude-opus-5-5

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[augmentcode]

🤖 Augment PR Summary

Summary: Refactors pfn compile-fail tests for top-to-bottom readability without changing library code.

• Moves expected and optional rejection tests into tests/pfn/reject/.
• Replaces numeric preprocessor matrices with named LIBFN_REJECT_ guards.
• Places positive controls beside their corresponding rejection scenarios.
• Adds a reusable refusing callback fixture with invocability assertions.
• Builds each source without rejection guards as a positive-control object target.
• Discovers and validates scenario guards during CMake configuration.
• Registers each guarded build as a CTest WILL_FAIL test.
• Uses a shared resource lock to serialize builds within the test tree.
• Preserves 41 rejection scenarios per enabled mode and updates contributor guidance and test references.

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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[augmentcode]

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