fix: gursoy-atun-layout.cpp modernized and explicitely checks correctness

[Becheler]

Before submitting

• This PR targets the develop branch.
• I searched for an existing PR or issue covering the same change.
• My contribution is licensed under the Boost Software License 1.0.

Type of change

• Bug fix
• New feature or API addition
• Refactor (no behavior change)
• Documentation
• Build, CI, or tooling
• Other (specify below)

Does this PR introduce a breaking change?

• Yes (describe migration impact below)
• No

What this PR does

• deterministic test
• modernized test
• instead of print-only, guarantees invariants available
• checks weighted/unweighted version
• uses fully positional overload for weighted case

Motivation

Testing

Checklist

• Existing tests pass (b2 in the test/ directory).
• New behavior is covered by a test, or this is a docs / build / refactor change.
• Documentation was updated if user-facing behavior changed.
• No new compiler warnings on the platforms I built against.

[github-actions]

Boost dependency footprint vs develop (auto-generated).
PR run 31474916928 vs develop run 31467777307 (8c958ac0fb).

Header-inclusion weights (graph files pulling each direct dependency in):

No header-inclusion-weight changes.

Transitive Boost modules: 68 → 68 (0)

[github-actions]

Compiler-warning counts vs develop (auto-generated).
PR run 31474916946 vs develop run 31467777283 (8c958ac0fb).

Job	Baseline	After	Delta
macos (clang, 14)	681	680	-1
macos (clang, 17)	641	641	0
macos (clang, 20)	641	641	0
ubuntu (clang-19, 14)	681	680	-1
ubuntu (clang-19, 17)	641	641	0
ubuntu (clang-19, 20)	641	641	0
ubuntu (clang-19, 23)	641	641	0
ubuntu (gcc-14, 14)	827	827	0
ubuntu (gcc-14, 17)	949	949	0
ubuntu (gcc-14, 20)	949	949	0
ubuntu (gcc-14, 23)	949	949	0
windows_msvc_14_3 (msvc-14.3)	962	959	-3

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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