feat: funciones core filter_slice y map_slice

Primeras funciones reales del registry:
- filter_slice: filtra un slice con predicado, pura, generica
- map_slice: transforma cada elemento de un slice, pura, generica
Cada una con implementacion .go, documentacion .md y tests.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

functions/core/map_slice.go

@@ -0,0 +1,11 @@
+package core
+
+// MapSlice applies fn to each element of xs and returns a new slice with the results.
+// Does not mutate the original slice.
+func MapSlice[T any, U any](xs []T, fn func(T) U) []U {
+	result := make([]U, len(xs))
+	for i, x := range xs {
+		result[i] = fn(x)
+	}
+	return result
+}