Make the output of girth more like the original.

[ascherer]

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

It makes the output of the 'girth' example program more like the original SGB program, both in normal results and in error situations.

Motivation

Running ./girth I noticed the output differences compared to the original girth.

Testing

g++ -I /usr/include/sgb -L /usr/lib/x86_64-linux-gnu/sgb -lgb girth.cpp -o girth

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 33395747748 vs develop run 33385637373 (768ca44efd).

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

No header-inclusion-weight changes.

Transitive Boost modules: 66 → 66 (0)

[github-actions]

Compiler-warning counts vs develop (auto-generated).
PR run 33395747943 vs develop run 33385637457 (768ca44efd).

Job	Baseline	After	Delta
macos (clang, 14)	647	647	0
macos (clang, 17)	610	610	0
macos (clang, 20)	610	610	0
ubuntu (clang-19, 14)	647	647	0
ubuntu (clang-19, 17)	610	610	0
ubuntu (clang-19, 20)	610	610	0
ubuntu (clang-19, 23)	610	610	0
ubuntu (gcc-14, 14)	795	795	0
ubuntu (gcc-14, 17)	931	931	0
ubuntu (gcc-14, 20)	931	931	0
ubuntu (gcc-14, 23)	931	931	0
windows_msvc_14_3 (msvc-14.3)	952	952	0

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.