refactor: drop direct dependency on Boost.SmartPtr

[Becheler]

General hygiene related to Boost.SmartPtr

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

• Remove dead includes
• Refactor smart pointers from Boost to STL
• Removed the dead r_c_shortest_paths_label base boost::enable_shared_from_this. shared_from_this() was never called.
• Deleted 4 free comparison operators for shared_ptr<label> that dereferenced the pointer to sort (by label). With std::shared_ptr those would compete with STL comparison (by pointer).
• Replaced them with comparators dereferencing the pointer: a small splabel_greater functor for the priority_queue, and a lambda for list::sort.

Motivation

Boost.SmartPtr does not have heavy dependencies (config, assert, throw_excpetion and core), but dead includes and C++14 STL is proper hygiene: https://github.com/cppalliance/cppalliance.github.io

Testing

Unchanged tests in CI

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 29821847603 vs develop run 29738450065 (ca4c9dfed1).

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

Dependency	develop	PR	Δ
smart_ptr	17	0	-17

Transitive Boost modules: 68 → 68 (0)

[github-actions]

Compiler-warning counts vs develop (auto-generated).
PR run 29821847581 vs develop run 29738450095 (ca4c9dfed1).

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

[jeremy-murphy]

Very minor request, so I'm approving now.