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.md

@@ -0,0 +1,32 @@
+---
+name: reduce
+kind: function
+lang: go
+domain: core
+version: "1.0.0"
+purity: pure
+signature: "func Reduce[T, U any](slice []T, initial U, f func(U, T) U) U"
+description: "Reduce un slice a un unico valor aplicando una funcion acumuladora de izquierda a derecha."
+tags: [slice, functional, generic, fold]
+uses_functions: []
+uses_types: []
+returns: []
+returns_optional: false
+error_type: ""
+imports: []
+tested: false
+tests: []
+test_file_path: ""
+file_path: "functions/core/reduce.go"
+---
+
+## Ejemplo
+
+```go
+sum := Reduce([]int{1, 2, 3, 4}, 0, func(acc, x int) int { return acc + x })
+// sum = 10
+```
+
+## Notas
+
+Funcion pura generica. Implementacion en devfactory/core.