Strings and frequency maps

Implement Prototype(loaded, requested string) map[rune]int by rescanning and Solve with frequency maps. Apply provided FoldAlphabetic to each complete string, then return only positive requested-minus-loaded counts. Empty inputs are valid. Rune keys replace single-code-point string keys. Go maps have no iteration order: consumers must sort keys for display; map order is not part of this variant's API. No further normalization. The helper supplies Unicode full case folding and alphabetic filtering so ß matches SS; it is not the counting solution.

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.