feat: add NextN method to SpecSchedule

[walle250ai]

Summary

• Add NextN(t time.Time, n int) []time.Time to SpecSchedule
• Returns the next n activation times after t, in ascending order
• Reuses the existing Next method internally — no duplicate logic
• Returns an empty slice when n <= 0; stops early if Next returns zero (no valid time within 5 years)

Test plan

• n=0 and negative n return empty slice
• n=1 returns the correct single next time
• n=3 with */1 * * * * * returns 3 consecutive seconds
• Unsatisfiable schedule (Feb 30) returns fewer than n results
• Results are verified to be strictly increasing

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