refactor: drops boost::multiprecision for boost::int128

[Becheler]

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

Replace Boost.Multiprecision (25 dependencies) int128 with Boost.Int128 (0 dependency).

Motivation

Better type, faster, less dependencies.

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 32753442710 vs develop run 32724296748 (a6fc56bba7).

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

Dependency	develop	PR	Δ
multiprecision	1	0	-1
int128	0	1	+1

Transitive Boost modules: 66 → 66 (0)

• added: charconv, int128
• removed: math, multiprecision

[github-actions]

Compiler-warning counts vs develop (auto-generated).
PR run 32753442826 vs develop run 32724296558 (a6fc56bba7).

Job	Baseline	After	Delta
macos (clang, 14)	607	607	0
macos (clang, 17)	570	570	0
macos (clang, 20)	570	570	0
ubuntu (clang-19, 14)	607	607	0
ubuntu (clang-19, 17)	570	570	0
ubuntu (clang-19, 20)	570	570	0
ubuntu (clang-19, 23)	570	570	0
ubuntu (gcc-14, 14)	768	768	0
ubuntu (gcc-14, 17)	904	904	0
ubuntu (gcc-14, 20)	904	904	0
ubuntu (gcc-14, 23)	904	904	0
windows_msvc_14_3 (msvc-14.3)	938	938	0

[codecov]

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✅ All modified and coverable lines are covered by tests.

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