Honor Ractor callbacks in direct mode

[OskarEichler]

Summary

Validate Ractor callback usage before worker-count dispatch and execute the configured callback in zero-worker direct mode.

External map, indexed map, each, and invalid-combination models pass on Ruby 4.0.6 and 3.2.11. Diff and syntax checks pass. Source only; repository tests are unchanged.

[grosser]

I think I fixed that with some previous PRs already, lmk if any work remains

[OskarEichler]

Confirmed against current master: the empty-input/direct path now replaces the block with ractor_block(options) before calling work_direct, so it preserves the ractor: [Class, :method] contract and its validation without the additional dispatch code from this PR. No further change is needed here. Thanks for checking.