Add adjacency_matrix serialization support.

[varunsha30]

Important

New algorithms and API additions need a design issue first. Such a PR is not reviewed until an issue has settled the API and frozen the scope. PRs whose scope grows under review go stale. See How To Contribute an Algorithm.

Motivation

Part of #473

What this PR does

Add adjacency_matrix serialization support.

Before submitting

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

Kind of change

Tick exactly one. Each kind is reviewed differently, so mixing them in one PR makes the diff difficult to review (why).

• Reformat (layout only, no token added or removed)
• Refactor (no observable behavior change)
• Behavior change (observable to users of the library)
• No code change (documentation, build, CI, or tooling)

Change category

• Bug fix
• New algorithm or API addition
• Deprecation or removal
• Performance
• Tests
• Documentation
• Build, CI, or tooling
• Other (specify below)

[github-actions]

Boost dependency footprint vs develop (auto-generated).
PR run 38753564193 vs develop run 38291727273 (07e865fdfa).

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

Dependency	develop	PR	Δ
core	14	15	+1
type_traits	41	42	+1

Transitive Boost modules: 47 → 47 (0)

[github-actions]

Compiler-warning counts vs develop (auto-generated).
PR run 38753564336 vs develop run 38291727442 (07e865fdfa).

Job	Baseline	After	Delta
macos (clang, 14)	253	257	+4
macos (clang, 17)	252	256	+4
macos (clang, 20)	252	256	+4
ubuntu (clang-19, 14)	253	257	+4
ubuntu (clang-19, 17)	252	256	+4
ubuntu (clang-19, 20)	252	256	+4
ubuntu (clang-19, 23)	252	256	+4
ubuntu (gcc-14, 14)	349	355	+6
ubuntu (gcc-14, 17)	345	351	+6
ubuntu (gcc-14, 20)	345	351	+6
ubuntu (gcc-14, 23)	345	351	+6
windows_msvc_14_3 (msvc-14.3)	937	937	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.