refactor: decouple serialization

[Becheler]

Remove dependency to Boost.Serialization

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

• Phase out of serialization nvp utilities to boost.core.serialization
• Use a forward declaration trick to decouple BGL from Serialization.

Motivation

Boost.Serialization is one of the heaviest Boost.Library along with Spirit: 22 direct dependencies, 55 total, effectively dragging most of Boost in default build: https://alandefreitas.github.io/boostdep_graph/libs/serialization.html

In contrast, Boost.Core.Serialization has 3 dependencies: https://alandefreitas.github.io/boostdep_graph/libs/core.html

We switch to Boost.Core.Serialization dependencies and use a forward declaration trick inspired by Peter Dimov uuid trick to remove direct coupling on Boost.Serialization (the user who needs Serialization will pull its headers anyway): https://github.com/boostorg/uuid/blob/2aa25a3afe023c5324d1b13015a65f67bfaef08c/include/boost/uuid/uuid.hpp#L344-L357

Some Serialization stuff for property map is behind an MPI guard: this is actually Boost.Parallel territory, and looks like a remnant of modularization BGL/PBGL. This should be exported to PBGL rather than polluting BGL, but it's a cross-repo PR and outside of the current scope.

Testing

The current test is unchanged, we just add a static assertion to verify the tracking level is unchanged (it exists to guarantee that non-const graphs can be serialized, removing it would cause a compilation error deep inside Serialization if the user tries to serialize a non-const graph).

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 29729661107 vs develop run 29726826217 (e6ba37cdd4).

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

Dependency	develop	PR	Δ
serialization	2	1	-1
type_traits	40	41	+1
core	13	15	+2

Transitive Boost modules: 69 → 69 (0)

[github-actions]

Compiler-warning counts vs develop (auto-generated).
PR run 29729661111 vs develop run 29726826190 (e6ba37cdd4).

Job	Baseline	After	Delta
macos (clang, 14)	4	4	0
macos (clang, 17)	4	4	0
macos (clang, 20)	4	4	0
ubuntu (clang-19, 14)	4	4	0
ubuntu (clang-19, 17)	4	4	0
ubuntu (clang-19, 20)	4	4	0
ubuntu (clang-19, 23)	4	4	0
ubuntu (gcc-14, 14)	10	10	0
ubuntu (gcc-14, 17)	10	10	0
ubuntu (gcc-14, 20)	10	10	0
ubuntu (gcc-14, 23)	10	10	0
windows_msvc_14_3 (msvc-14.3)	971	971	0