Fix inconsistent shift() behavior across DST transitions (#1170)

[IDEAS247]

Pull Request Checklist

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• 🧪 Added tests for changed code.
• 🛠️ All tests pass when run locally (run tox or make test to find out!).
• 🧹 All linting checks pass when run locally (run tox -e lint or make lint to find out!).
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Description of Changes

Closes: #1170

Problem

Arrow.shift() applies hour/minute/second shifts as wall-clock arithmetic,
so results become inconsistent across a Daylight Saving Time transition.
For example, on the US spring-forward date:

x = arrow.Arrow(2023, 3, 12, tzinfo="America/Los_Angeles")
x.shift(hours=2) == x.shift(hours=3)  # True — should never happen

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (2224255) to head (94c4416).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #1365   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           10        10           
  Lines         2315      2325   +10     
  Branches       358       361    +3     
=========================================
+ Hits          2315      2325   +10     

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