Sliding windows with counts

Implement Prototype(labels []string, limit int) (int,error) by enumerating ranges; Solve uses one moving window and frequency map for expected O(n) time. Return the longest contiguous range with at most limit exact distinct labels. Empty input or zero limit returns zero. Negative limit returns error before mutation. Preserve input.

Edit exercise.go; retain the public types and signatures. prototype and final are separate stages. The executable tests recover panics as failures, report named checks, and exit nonzero on any failure. A successful run must execute at least one check. Tests use logical time or explicit synchronization where relevant.