Fix schedule dispatch at exact timestamp boundaries.
- Preserve persisted fractional precision when selecting due schedules, including SQLite's text timestamp comparison.
- Keep future deadlines pending and repeated ticks idempotent with the original occurrence identity.
- No public API, database schema, payload protocol or compatibility change.
Fix: #505 / #506. Release metadata: #507.
Full scheduler-fix main CI passed MySQL, PostgreSQL, MariaDB, Laravel upgrade, coverage and regression checks. Publication verification and affected-consumer updates are tracked in #505.