Add CalculateCyclesInWindow function

[TyostoKarry]

Adds window-based cycle calculation to the core logic, returning both complete cycle count and remaining overflow duration.

Changes
internal/cycle/cycle.go_

• CalculateCyclesInWindow — calculates complete cycles and remaining overflow duration between two times, handles next-day wake by adding 24h when to is not after from

internal/cycle/cycle_test.go

• Tests covering exact cycles, with/without buffer, next-day wake, overflow only, midnight boundary, and same from/to edge case

Notes

• Returns (int, time.Duration) — overflow is intentionally exposed for use by the output layer to indicate how far into a partial cycle the wake time falls
• Same from/to input results in a 24h window (16 cycles) — this should be guarded against at the CLI layer