Preserve runtime prototype keyword rest names

[OskarEichler]

Summary: retain named keyword-rest parameters in runtime prototypes while continuing to normalize the anonymous forwarding sentinel.

Verification: focused baseline/fixed reproduction, combined RBS 4.2.0 consumer models, and RuboCop (738 files, zero offenses). No tests are added in this PR.

Compatibility: no public API removal or dependency/version change.

[OskarEichler]

The implementation correctly changes runtime prototype output for named keyword-rest parameters, but the existing focused expectations still asserted the old anonymous form. I updated both merge and non-merge expectations in f7fa06f. The focused runtime prototype suite now passes (18 tests, 27 assertions, 1 expected omission), targeted RuboCop is clean, and git diff --check passes. The remaining ruby-head stdlib errors are shared upstream drift and unrelated to this change.