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/flatten.md

@@ -0,0 +1,32 @@
+---
+name: flatten
+kind: function
+lang: go
+domain: core
+version: "1.0.0"
+purity: pure
+signature: "func Flatten[T any](slices [][]T) []T"
+description: "Aplana un slice de slices en un unico slice."
+tags: [slice, functional, generic]
+uses_functions: []
+uses_types: []
+returns: []
+returns_optional: false
+error_type: ""
+imports: []
+tested: false
+tests: []
+test_file_path: ""
+file_path: "functions/core/flatten.go"
+---
+
+## Ejemplo
+
+```go
+result := Flatten([][]int{{1, 2}, {3, 4}, {5}})
+// result = [1, 2, 3, 4, 5]
+```
+
+## Notas
+
+Funcion pura generica. Implementacion en devfactory/core.