refactor: Getting rid of clippy slicing

[SimonRastikian]

Reworks near/threshold-signatures#122
Remove the blanket #![allow(clippy::indexing_slicing)]
In some spots I added input validation and converted iterators to prevent using indexing

[claude]

PR title format: This PR is missing a conventional commit type prefix. Since it fixes clippy linting warnings without changing functionality, the type should be style:

Suggested title: style: fix clippy slicing warnings

[claude]

Code Review

No critical issues found. The PR cleanly replaces blanket #![allow(clippy::indexing_slicing)] with:

• Scoped #[allow(...)] with safety invariant comments (strobe.rs)
• Iterator/zip-based loops instead of index-based loops
• Input validation (len < N checks, .first(), .get()) before access

The zip-based rewrites correctly preserve original loop semantics — zip stops at the shorter iterator, which matches the original 0..N bounds after the added length guards.

✅ Approved

[claude]

PR title type suggestion: This PR should use a conventional commit type prefix. Since it's restructuring code to address clippy warnings, the type should be refactor:.
Suggested title: refactor: address clippy slicing warnings

[claude]

Code Review

No critical issues found. The changes correctly:

• Replace blanket #![allow(clippy::indexing_slicing)] with scoped #[allow(...)] annotations with safety invariant comments (strobe.rs)
• Convert index-based loops to iterator/zip-based loops throughout
• Add input length validation (len < N checks, .first(), .get(), split_first()) before access
• Return ProtocolError::AssertionFailed on validation failures rather than panicking

The zip-based rewrites correctly preserve original loop semantics — zip stops at the shorter iterator, which matches the original 0..N bounds, and the added length guards ensure the receiving vectors are at least N long.

✅ Approved

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Reviewed with Claude Code

[netrome]

Nice stuff, thanks for fixing. Would be nice to use multizip to make the zip chain a bit prettier but not a hard blocker.

[netrome]

Not sure I've ever seen this many zips in the same expression 😂

Perhaps worth including itertools (we already have it in the workspace) and use multizip https://docs.rs/itertools/latest/itertools/fn.multizip.html