Rcsp new interface overloads

[andreacassioli]

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

Motivation

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 30041728548 vs develop run 29836931414 (7630d5cdf9).

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

No header-inclusion-weight changes.

Transitive Boost modules: 68 → 68 (0)

[github-actions]

Compiler-warning counts vs develop (auto-generated).
PR run 30041728537 vs develop run 29836931410 (7630d5cdf9).

Job	Baseline	After	Delta
macos (clang, 14)	4	8	+4
macos (clang, 17)	4	8	+4
macos (clang, 20)	4	8	+4
ubuntu (clang-19, 14)	4	8	+4
ubuntu (clang-19, 17)	4	8	+4
ubuntu (clang-19, 20)	4	8	+4
ubuntu (clang-19, 23)	4	8	+4
ubuntu (gcc-14, 14)	10	17	+7
ubuntu (gcc-14, 17)	10	17	+7
ubuntu (gcc-14, 20)	10	17	+7
ubuntu (gcc-14, 23)	10	17	+7
windows_msvc_14_3 (msvc-14.3)	971	979	+8