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/compose2.md

@@ -0,0 +1,34 @@
+---
+name: compose2
+kind: function
+lang: go
+domain: core
+version: "1.0.0"
+purity: pure
+signature: "func Compose2[A, B, C any](f2 func(B) C, f1 func(A) B) func(A) C"
+description: "Compone dos funciones de derecha a izquierda. compose2(g, f)(x) = g(f(x))."
+tags: [functional, composition, generic]
+uses_functions: []
+uses_types: []
+returns: []
+returns_optional: false
+error_type: ""
+imports: []
+tested: false
+tests: []
+test_file_path: ""
+file_path: "functions/core/compose2.go"
+---
+
+## Ejemplo
+
+```go
+double := func(x int) int { return x * 2 }
+toString := func(x int) string { return fmt.Sprintf("%d", x) }
+doubleStr := Compose2(toString, double)
+result := doubleStr(5) // "10"
+```
+
+## Notas
+
+Funcion pura generica. Implementacion en devfactory/core.