feat: implement musical time conversion functionality

- Add MusicalTime dataclass with cycle, hierarchical position, and fractional beat
- Add Meter.get_musical_time() method with optional reference level support
- Support arbitrary hierarchical depth (beat, subdivision, sub-subdivision, etc.)
- Use pulse-based fractional calculation to handle rubato and irregular timing
- Add comprehensive test suite with 18 test cases covering all functionality
- Update package exports for MusicalTime class
- Implement specification from docs/musical-time-spec.md

Features:
- Convert real time to musical position within meter boundaries
- Reference levels allow fractional calculation at any hierarchy level
- Handles complex meters like [3, 2, 4] with proper level naming
- Returns False for out-of-bounds times, MusicalTime for valid times
- String representations: compact (C0:2.1+0.500) and readable formats

Author: jon-myers