fix(cosh-ng): swap chunks_exact for as_chunks

[jfeng18]

Why

Clippy 1.98 (rolling stable toolchain) ships the new chunks_exact_to_as_chunks lint. It fires on pre-existing decode_hex code in cosh-platform, turning the fast-checks job red on PRs that run stable clippy (e.g. #2708).

What changed

Swapped .chunks_exact(2) for .as_chunks::<2>().0.iter() in decode_hex — the exact form clippy suggests. Semantics-preserving: both iterate full 2-byte chunks and drop the tail remainder; the length-parity check above guarantees the remainder is always empty here.

Related issue

no-issue: pre-existing code flagged by a new clippy lint, not a user-visible bug. Unblocks the #2708 fast-checks job. Attribution: Fixes: 1a935895a391e9714e000fab00c3602a6580cd79 ("feat(cosh-ng): [core,shell] add audit log") in the commit body per repo convention.

User / Agent impact

None. Internal API-equivalent refactor of a private hex decoder.

Risk and compatibility

• Public CLI, API, configuration, or documented behavior changed
• Privileged or security-sensitive behavior changed
• Cross-component contract changed
• Migration or rollback guidance is needed

Low risk: as_chunks stabilized in Rust 1.88 and cosh-ng CI pins 1.89, so the pinned-toolchain job compiles unchanged.

Validation

• Reproduced pre-fix: stable 1.98 cargo clippy -p cosh-platform --all-targets -- -D warnings fails at query.rs:629; post-fix exit 0
• ECS (Linux, RUSTUP_TOOLCHAIN=1.89.0): cargo clippy -p cosh-platform --all-targets -- -D warnings clean; cargo test -p cosh-platform 311 passed / 0 failed
• cargo fmt --all --check clean under both 1.89 and stable 1.98
• Equivalence harness (rustc 1.89, not committed): old and new decode_hex agree on all 484 hex-pair inputs plus 10 boundary inputs (empty, odd-length, non-hex)

Documentation and rollback

No docs change. Rollback = revert the single commit.

[qoderai]

本次修改范围内未发现功能或安全缺陷。唯一变更为遵循新 clippy lint 的等价重构，无需额外改动。

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[KaiLongZhou]

Code Review: fix(cosh-ng): swap chunks_exact for as_chunks

Verdict: LGTM ✅

Correctness

变更语义等价，分析如下：

• chunks_exact(2) 产生 &[u8] 迭代器（每片长度 2，丢弃余数）
• as_chunks::<2>().0.iter() 产生 &[u8; 2] 迭代器（同样丢弃余数）
• 上层 value.len() & 1 == 1 的奇偶校验保证余数始终为空，两种写法处理完全相同的输入
• pair[0] / pair[1] 索引在 &[u8] 和 &[u8; 2] 上行为一致

变换是纯机械性的 clippy 建议替换，无逻辑变更。

观察（非阻塞）

1. MSRV 隐式提升：workspace Cargo.toml 声明 rust-version = "1.74"，但 as_chunks 在 Rust 1.88 才稳定。虽然 CI 实际使用 stable（当前 1.98），但如果有外部用户按 MSRV 1.74 编译 cosh-platform 会失败。建议后续统一更新 rust-version 字段或确认 MSRV 仅为 aspirational。

2. 同 lint 其他触发点：仓库中还有两处 chunks_exact(2) 可能在同一 clippy 版本下被标记：

   • cosh-core/src/tool/grep.rs:511 — decode_utf16_bytes 中的 chunks_exact(2)
   • cosh-shell/src/recommendation/personal_crypto.rs:259 — 测试辅助 decode_hex 中的 chunks_exact(2)

   本 PR 仅修复 cosh-platform，如果 fast-checks 只跑 cargo clippy -p cosh-platform 则足够；若后续 workspace 级别 clippy 也报错，需要追加修复。

3. 可读性：.as_chunks::<2>().0.iter() 比 .chunks_exact(2) 稍显晦意（.0 是 tuple struct access），但这是 clippy 推荐形式且提供了更强的静态保证（[u8; 2] vs [u8]），可接受。

总结

单文件、单函数、机械性 lint 修复，风险极低。验证充分（PR 描述中列出了等价性测试）。LGTM。

[jfeng18]

Thanks for the thorough review and the LGTM!

Confirming the follow-up plan for the non-blocking observations:

• MSRV declaration alignment: bump workspace rust-version from 1.74 to 1.88 to match the as_chunks stabilization.
• The two remaining chunks_exact(2) sites — cosh-core/src/tool/grep.rs:511 and cosh-shell/src/recommendation/personal_crypto.rs:259 — will be swapped in the same follow-up PR.

It is being prepared right now and should land shortly.

One small ask: you are one of the requested reviewers, and the branch ruleset requires an approval before this PR can merge — would you mind converting your verbal LGTM into a formal review approval when convenient? No rush, just so the PR does not sit idle on that requirement. Thanks!