Compare and hash fixed-offset timezones by UTC offset

[Arborsen]

Fixes #466.

Changes

• Compare concrete datetime.timezone objects using their native offset-based equality.
• Hash fixed-offset timezones by utcoffset(None), so different offsets are distinguishable in sets and unordered comparisons, while equal offsets with different display names remain equal.
• Add 23 regression/control cases for root/nested values, ordered/unordered lists, sets, UTC aliases, negative and microsecond offsets, and datetime truncation options.

This keeps timezone objects in change reports for in-memory Delta replay. Custom tzinfo and ZoneInfo behavior is outside this change.

Validation

• Focused datetime/hash tests: 142 passed.
• Full suite including slow tests: 1,332 passed, 8 skipped; 95% total coverage.
• Original reproduction and controls: 10 passed.
• Fatal lint checks and git diff --check: passed.
• Pyright reports the same 50 errors as unchanged dev; no new diagnostics.
• Tested with Python 3.12.14.

Known limitation for review

The timezone hash follows the existing scalar hashing convention, which shares a namespace with user strings. Consequently, DeepDiff([timezone.utc], ['timezone:0:00:00'], ignore_order=True) returns an empty diff with this change, whereas the baseline reports a type change. Equivalent collisions already exist for other scalars (for example, 1 and 'int:1'). This patch does not redesign hash domain separation; maintainer guidance on the intended scope is welcome.

Existing timezone serialization requirements are unchanged: JSON output needs an appropriate default_mapping, and persisted Delta loading may require safe_to_import={'datetime.timezone'}.