feat: 21 funciones core — composicion funcional y operaciones de slice

Composicion: Pipe2, Pipe3, Compose2, Identity, Curry2, Uncurry2, Partial2,
Flip, Const. Slices: Find, FindIndex, Any, All, GroupBy, Flatten,
FlatMapSlice, Unique, Zip, Reduce, Take, Drop.
Stubs que documentan devfactory/core para el registry.

functions/core/reduce.go

@@ -0,0 +1,9 @@
+package core
+
+// Implementation: github.com/lucasdataproyects/devfactory/core
+
+// Reduce reduce un slice a un unico valor aplicando una funcion acumuladora.
+func Reduce[T, U any](slice []T, initial U, f func(U, T) U) U {
+	// stub
+	return initial
+}