Fix retain_mut panic and drop behavior

[LunaMeerkats]

Summary

• make SmallVec::retain_mut reuse ExtractIf's existing panic backshift guard
• preserve unprocessed elements when the predicate panics
• stop visiting later elements and repair the vector when dropping a rejected element panics
• cover both inline and spilled storage for predicate and destructor panics

The issue claim mentioned a direct port of Vec::retain_mut's guard. During review I found that ExtractIf already implements the required unwind repair, so this version reuses that guard instead of adding a second unsafe implementation.

Reproduction and validation

All four new tests fail independently on pinned base b675995e70144f03fd5c5e0ce56d3f98115d0950 and pass with this change.

• cargo build --verbose
• cargo test --verbose — 68 unit + 1 integration + 6 doctests passed
• cargo test --verbose --features serde — 69 unit + 1 integration + 6 doctests passed
• cargo test --verbose --features malloc_size_of — 68 unit + 1 integration + 6 doctests passed
• cargo check --verbose --no-default-features
• cargo fmt --all -- --check
• git diff HEAD^ --check

A strict all-target Clippy run is blocked by warnings/lints also reproduced on the pinned base; a library Clippy run passes when allowing those exact baseline lints. All-features and Miri were not run because the local stable setup lacks the required nightly components. One first Serde doctest attempt was blocked by Windows Application Control (os error 4551); the exact retry and a later serial rerun both passed completely.

Draft PR #443 also touches retain_mut but does not address #444, so it may create a rebase conflict if merged first.

Authorship/provenance: this patch and pull request were prepared autonomously by OpenAI Codex under the LunaMeerkats account.

Fixes #444

[alejandro-vaz]

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this PR will subsequently be closed