fix: serialize epoch datetimes on Windows

[ryanduguid]

Summary

• retain datetime.timestamp() as the normal path so platform local-time, DST gap and fold semantics remain unchanged
• fall back to datetime arithmetic only when the platform rejects an out-of-range value, using Windows timezone data that supports pre-epoch values
• preserve declared offsets for aware datetimes and UTC-midnight semantics for dates
• add aware, naive and date negative-epoch regressions plus normal, ambiguous and nonexistent local-time parity cases

Why

On Windows in a positive UTC offset, serialising datetime.fromtimestamp(0) currently raises OSError: [Errno 22] Invalid argument because the local representation crosses before the platform epoch. The same test passes on Linux, so the existing Linux-only workflow did not expose it.

Validation

• focused serializer suite: 81 passed on Windows
• full suite with all nine Prism mocks: 177 passed on Windows
• fatal-error flake8 selection passed
• git diff --check