Fix nightly CI failures from unused ARM feature gates

[Copilot]

Summary

Fix nightly CI failures caused by unconditional arm_target_feature crate feature gates being enabled for non-ARM unstable test builds.

Changes

• restrict arm_target_feature to all(feature = "unstable", target_arch = "arm") in hex-simd, base64-simd, base32-simd, and unicode-simd
• keep ARM unstable behavior unchanged while avoiding nightly unused_features warnings on x86, wasm, and other CI targets
• validate with the repository's nightly develop checks, stable release workspace tests, and targeted nightly unstable tests for the affected crates

Validation

• cargo test
• cargo check --all-targets
• cargo +nightly fmt --all -- --check
• cargo +nightly clippy -- -D warnings
• cargo +nightly build --release --workspace --tests
• cargo test --release --workspace --exclude simd-benches
• cargo +nightly test --target x86_64-unknown-linux-gnu -p hex-simd --lib --tests --no-default-features --features unstable
• cargo +nightly test --target x86_64-unknown-linux-gnu -p base64-simd --lib --tests --no-default-features --features unstable
• cargo +nightly test --target x86_64-unknown-linux-gnu -p base32-simd --lib --tests --no-default-features --features unstable
• cargo +nightly test --target x86_64-unknown-linux-gnu -p unicode-simd --lib --tests --no-default-features --features unstable

Security Summary

• Ran automated code review: no comments
• Ran CodeQL checker: 0 alerts

[Copilot]

Pull request overview

Fixes nightly CI failures triggered by enabling the arm_target_feature nightly feature gate on non-ARM targets when building the SIMD crates with --features unstable.

Changes:

• Gate feature(arm_target_feature) behind all(feature = "unstable", target_arch = "arm") to avoid unused_features warnings on non-ARM targets.
• Apply the same gating consistently across hex-simd, base64-simd, base32-simd, and unicode-simd.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
crates/unicode-simd/src/lib.rs	Restricts arm_target_feature to unstable ARM builds only.
crates/hex-simd/src/lib.rs	Restricts arm_target_feature to unstable ARM builds only.
crates/base64-simd/src/lib.rs	Restricts arm_target_feature to unstable ARM builds only.
crates/base32-simd/src/lib.rs	Restricts arm_target_feature to unstable ARM builds only.

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