fix: fields initialized

[Becheler]

#496

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

Initalize fields to solve multiple -Wmissing-field-initializers

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 31248398546 vs develop run 31207793080 (198303a7ca).

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 31248398536 vs develop run 31207793516 (198303a7ca).

Job	Baseline	After	Delta
macos (clang, 14)	681	668	-13
macos (clang, 17)	641	628	-13
macos (clang, 20)	641	628	-13
ubuntu (clang-19, 14)	681	668	-13
ubuntu (clang-19, 17)	641	628	-13
ubuntu (clang-19, 20)	641	628	-13
ubuntu (clang-19, 23)	641	628	-13
ubuntu (gcc-14, 14)	827	800	-27
ubuntu (gcc-14, 17)	949	922	-27
ubuntu (gcc-14, 20)	949	922	-27
ubuntu (gcc-14, 23)	949	922	-27
windows_msvc_14_3 (msvc-14.3)	971	971	0

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.44%. Comparing base (e8e3f01) to head (198303a).
⚠️ Report is 1 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #553      +/-   ##
===========================================
- Coverage    92.45%   92.44%   -0.02%     
===========================================
  Files          392      392              
  Lines        28214    28214              
  Branches      8008     8008              
===========================================
- Hits         26086    26082       -4     
- Misses        2024     2027       +3     
- Partials       104      105       +1     

Files with missing lines	Coverage Δ
test/graphviz_test.cpp	96.44% <ø> (ø)

... and 4 files with indirect coverage changes

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