resolve : mark repr_simd as internal

[lms0806]

I changed repr_simd to internal and changed the position to feature-group-start: internal feature gates.

close #154034

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[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.