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/pipe3.md

@@ -0,0 +1,35 @@
+---
+name: pipe3
+kind: function
+lang: go
+domain: core
+version: "1.0.0"
+purity: pure
+signature: "func Pipe3[A, B, C, D any](f1 func(A) B, f2 func(B) C, f3 func(C) D) func(A) D"
+description: "Compone tres funciones de izquierda a derecha."
+tags: [functional, composition, generic, pipe]
+uses_functions: []
+uses_types: []
+returns: []
+returns_optional: false
+error_type: ""
+imports: []
+tested: false
+tests: []
+test_file_path: ""
+file_path: "functions/core/pipe3.go"
+---
+
+## Ejemplo
+
+```go
+add1 := func(x int) int { return x + 1 }
+double := func(x int) int { return x * 2 }
+toString := func(x int) string { return fmt.Sprintf("%d", x) }
+pipeline := Pipe3(add1, double, toString)
+result := pipeline(3) // "8"
+```
+
+## Notas
+
+Funcion pura generica. Implementacion en devfactory/core.