Fractional tweaks

[djc]

Followup from

• Support fractional durations (with precision safeguards, without any new dependency) #56

[Copilot]

Pull Request Overview

This PR refactors the duration parsing code by extracting unit parsing logic into a separate enum and improving code organization. The changes focus on making the parsing logic more maintainable while preserving existing functionality.

Key changes:

• Introduced a new Unit enum with FromStr implementation to replace string matching
• Refactored duration accumulation logic into a standalone function
• Reorganized method order within the Parser implementation

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