fix(hstr): avoid references to uninitialized bytes

[labor0-bot]

Description:

Fixes two soundness issues in hstr where mutable byte references were formed over uninitialized storage.

• Use an initialized 64-byte stack buffer in Atom::from_mutated_str, retaining the stack fast path and heap fallback for longer strings.
• Encode WTF-8 code points through Vec::spare_capacity_mut() and MaybeUninit::write, updating the vector length only after the exact encoded byte count is initialized.
• Document the safety invariants for retained unsafe operations.
• Add focused regression coverage for short, 64-byte, and over-64-byte ASCII case conversion and one- through four-byte code-point appends, including an unpaired surrogate.

Validation:

• cargo test -p hstr
• Focused Miri runs for both new regression tests
• cargo fmt --all
• cargo clippy --all --all-targets -- -D warnings

Related issue (if exists):

Closes #12083

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Latest commit: 21ffde8

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[codspeed-hq]

Merging this PR will not alter performance

✅ 200 untouched benchmarks
⏩ 61 skipped benchmarks¹

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_{Comparing fix/hstr-uninitialized-memory (21ffde8) with main (83ab4ed)}

Footnotes

1. 61 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩