feat: expose Duration field on Result struct

[AdamMagued]

Fixes #33

Expose Duration time.Duration on Result in when.go. Populate Result.Duration from rules.Context.Duration after rule evaluation.

Background:
Issue #33 requested ability to determine duration distance from reference dates or capture relative intervals without discarding time delta information. Rules for deadlines, past time, casual dates, and weekdays already accumulate deltas into rules.Context.Duration, but Result previously discarded this value.

Changes:

• Add Duration time.Duration field to Result struct in when.go.
• Assign res.Duration = ctx.Duration in Parser.Parse.
• Add test suite in when_test.go covering relative deadlines, past times, casual dates, static reference date offsets, absolute times, and custom duration rules.