Perf: Borrow Ut1Provider, O(log n) UT1 offset lookup, and faster EOP parsing

[FusRoman]

Summary

This PR reduces the overhead of UT1 conversions by:

• Borrowing Ut1Provider instead of moving it (no more large Vec clones on hot paths).
• Replacing the reverse linear scan in Epoch::ut1_offset with an O(log n) lookup using slice::partition_point, plus a fast-path when the query epoch is ≥ the last record.
• Making the EOP parser (Ut1Provider::from_eop_data) single-pass with no per-line allocation and a conditional sort (only sort if input is detected out of order).
• Adding ergonomic, zero-copy iteration helpers on Ut1Provider (iter, as_slice, and IntoIterator for &Ut1Provider).

These changes eliminate unnecessary cloning, reduce allocations, and make UT1 offset queries scale with the number of records.

Breaking change & Migration

This PR introduces a small breaking change: UT1 conversion functions now take a
borrowed provider (&Ut1Provider) instead of owning it. The migration is a
one-liner at each call site.

What changed

- pub fn from_ut1_duration(duration: Duration, provider: Ut1Provider) -> Self
+ pub fn from_ut1_duration(duration: Duration, provider: &Ut1Provider) -> Self

- pub fn to_ut1_duration(&self, provider: Ut1Provider) -> Duration
+ pub fn to_ut1_duration(&self, provider: &Ut1Provider) -> Duration

- pub fn to_ut1(&self, provider: Ut1Provider) -> Self
+ pub fn to_ut1(&self, provider: &Ut1Provider) -> Self

close #418

[gemini-code-assist]

Summary of Changes

Hello @FusRoman, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly enhances the performance and usability of UT1 conversions by transitioning to borrowed Ut1Provider instances, optimizing the UT1 offset lookup to logarithmic time, and improving the efficiency of EOP data parsing. It also introduces new ergonomic iteration helpers for better data access.

Highlights

• Borrowing Ut1Provider: The Ut1Provider is now passed by reference (&Ut1Provider) instead of by value in UT1 conversion functions (from_ut1_duration, to_ut1_duration, to_ut1). This change eliminates large Vec clones on hot paths, significantly reducing overhead.
• Optimized UT1 Offset Lookup: The ut1_offset lookup has been optimized from a linear scan to an O(log n) operation using slice::partition_point. A fast-path is also included for queries that are at or after the latest record, further improving performance for common scenarios.
• Efficient EOP Data Parsing: The Ut1Provider::from_eop_data parser has been refactored to be single-pass, avoid per-line allocations, and conditionally sort the data only if it's detected as out of order. This makes EOP data parsing much more efficient.
• Ergonomic Iteration Helpers: New ergonomic helpers (iter, as_slice, and IntoIterator for &Ut1Provider) have been added to Ut1Provider, providing zero-copy iteration and direct slice access to the underlying data.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (master@c0c0688). Learn more about missing BASE report.
⚠️ Report is 7 commits behind head on master.

Additional details and impacted files

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[gemini-code-assist]

Code Review

This is an excellent pull request that brings significant performance improvements to UT1 conversions. The change to borrow Ut1Provider instead of moving it is a great optimization, and the breaking change is well-documented. The new O(log n) lookup in ut1_offset and the single-pass EOP parser are impressive. The code is clean and the new iterator helpers improve the API's ergonomics. I have a couple of minor suggestions to further improve the robustness and conciseness of the EOP parser.

[ChristopherRabotin]

This is excellent, thank you!

Could you implement one of the recommendations by Gemini , and also change the version in the Cargo.toml to 4.2.0? That's just to prevent accidentally releasing this minor breaking change as the next 4.1.x version.

Thank you

[ChristopherRabotin]

Thanks @FusRoman!