Implement week/month/year units for stream PERIOD trigger with natural
boundary alignment and offset support.
Key changes:
- Parser: Add validation for natural time units (w/n/y) and offset parameter
- Time utilities: Add getDuration() support for week/month/year units
- TriggerTask: Implement window calculation with natural boundary alignment
- Week: align to Monday 00:00:00
- Month: align to 1st of month 00:00:00
- Year: align to Jan 1st 00:00:00
- Add offset support: PERIOD(1w, 1d) shifts window by 1 day
- Unit tests: Parser validation, time utilities, TriggerTask window calculation
- System tests: End-to-end tests for week/month/year units with offset
- Documentation: Update user manual with natural time unit examples
Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>
