fix(offset): accept "ut" / "UT" as a bare UTC timezone abbreviation

[0xSoftBoi]

Summary

• GNU `date -d` accepts bare `ut` and `UT` (Universal Time) as synonyms for UTC. `parse_datetime` rejected them with `InvalidInput` because `"ut"` was absent from the timezone abbreviation table.
• `gmt`/`GMT` already worked; this is a one-line addition of `"ut" => "+0"`.

Root cause

`timezone_name_to_offset` in `offset.rs` contained `"utc"` and `"gmt"` but was missing `"ut"`. Since the parser lowercases the input before matching, `UT` was also affected.

Changes

• `src/items/offset.rs`: add `"ut" => Ok("+0")` to the timezone lookup table; extend the `timezone_name_without_offset` unit test to include `"ut"`.
• `src/lib.rs`: add `test_bare_utc_timezone_abbreviations` covering all four forms (`ut`, `UT`, `gmt`, `GMT`) end-to-end.

Test Plan

• `cargo test` — all existing tests pass
• New regression test `test_bare_utc_timezone_abbreviations` confirms each abbreviation resolves to UTC offset +0

Fixes #280

[codspeed-hq]

Merging this PR will not alter performance

✅ 21 untouched benchmarks

---

_{Comparing 0xSoftBoi:fix/bare-timezone-abbreviations (e21fcf2) with main (4b412e0)}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.30%. Comparing base (4b412e0) to head (e21fcf2).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #287   +/-   ##
=======================================
  Coverage   99.30%   99.30%           
=======================================
  Files          20       20           
  Lines        3894     3903    +9     
  Branches      122      122           
=======================================
+ Hits         3867     3876    +9     
  Misses         26       26           
  Partials        1        1           

Flag	Coverage Δ
macos_latest	99.30% <100.00%> (+<0.01%)	⬆️
ubuntu_latest	99.30% <100.00%> (+<0.01%)	⬆️
windows_latest	14.01% <0.00%> (-0.04%)	⬇️

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[sylvestre]

please run cargo fmt

[tomagsx]

Review bump on this one too. The change itself is intentionally small and the tests/coverage are in place; the visible follow-ups are the formatting failure and the CodSpeed warning. If the approach looks acceptable from a maintainer standpoint, I can clean up the remaining CI nit quickly.