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/uncurry2.md

@@ -0,0 +1,35 @@
+---
+name: uncurry2
+kind: function
+lang: go
+domain: core
+version: "1.0.0"
+purity: pure
+signature: "func Uncurry2[A, B, C any](f func(A) func(B) C) func(A, B) C"
+description: "Transforma una funcion currificada en una funcion normal de dos argumentos."
+tags: [functional, generic, curry]
+uses_functions: []
+uses_types: []
+returns: []
+returns_optional: false
+error_type: ""
+imports: []
+tested: false
+tests: []
+test_file_path: ""
+file_path: "functions/core/uncurry2.go"
+---
+
+## Ejemplo
+
+```go
+curriedAdd := func(a int) func(int) int {
+    return func(b int) int { return a + b }
+}
+add := Uncurry2(curriedAdd)
+result := add(3, 5) // 8
+```
+
+## Notas
+
+Funcion pura generica. Implementacion en devfactory/core.