fix: correct ISO 8601 serialization for negative pendulum Duration values

[r266-tech]

Problem

The Duration.to_iso8601_string() method produces invalid ISO 8601 output for negative durations. For example:

Duration(days=-10).to_iso8601_string()   # returns '-P-10D' (invalid)
Duration(seconds=-10).to_iso8601_string() # returns '-PT-10S' (invalid)

The root cause is that individual component values (days, seconds, etc.) are negative, and the code also adds a - prefix, resulting in double negation in the output string.

This also causes data loss during model_copy(deep=True) since Pydantic serializes via to_iso8601_string() and the invalid string can't be properly deserialized.

Fixes #329

Fix

When the duration is negative, take absolute values of all components so the - prefix alone conveys the sign. The output is now valid ISO 8601:

Duration(days=-10).to_iso8601_string()   # returns '-P10D' ✓
Duration(seconds=-10).to_iso8601_string() # returns '-PT10S' ✓

Tests

Added 9 new tests covering:

• Negative duration serialization round-trip (days, seconds, hours, weeks, months, years, combined)
• Direct ISO 8601 string validation for negative durations

All 13517 tests pass (up from 13508).