clears gcc -Wsign-compare

[Becheler]

Refs #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

Fix -Wsign-compare (signed/unsigned comparison) warnings on gcc.

• include/boost/pending/detail/disjoint_sets.hpp: cast the property-map lookups to Vertex in the two link_sets asserts so they compare same-signedness (debug-only, stripped under NDEBUG).
• example/fibonacci_heap.cpp: changed the loop counter from int to std::size_t since it is compared against w.size().
• test/min_degree_empty.cpp: cast the loop index k to int in the BOOST_TEST comparison against the int-valued ordering.

Motivation

Refs #496

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]

Compiler-warning counts vs develop (auto-generated).
PR run 27953084325 vs develop run 27866501982 (6103fa5010).

Job	Baseline	After	Delta
macos (clang, 14)	46	46	0
macos (clang, 17)	41	41	0
macos (clang, 20)	41	41	0
ubuntu (clang-19, 14)	46	46	0
ubuntu (clang-19, 17)	46	46	0
ubuntu (clang-19, 20)	46	46	0
ubuntu (clang-19, 23)	46	46	0
ubuntu (gcc-14, 14)	28	22	-6
ubuntu (gcc-14, 17)	28	22	-6
ubuntu (gcc-14, 20)	28	22	-6
ubuntu (gcc-14, 23)	28	22	-6
windows_msvc_14_3 (msvc-14.3)	974	974	0