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/find.md

@@ -0,0 +1,33 @@
+---
+name: find
+kind: function
+lang: go
+domain: core
+version: "1.0.0"
+purity: pure
+signature: "func Find[T any](slice []T, predicate func(T) bool) Option[T]"
+description: "Devuelve el primer elemento del slice que cumple el predicado, envuelto en Option."
+tags: [slice, functional, generic, search]
+uses_functions: []
+uses_types: [option_go_core]
+returns: [option_go_core]
+returns_optional: false
+error_type: ""
+imports: []
+tested: false
+tests: []
+test_file_path: ""
+file_path: "functions/core/find.go"
+---
+
+## Ejemplo
+
+```go
+nums := []int{1, 2, 3, 4, 5}
+result := Find(nums, func(n int) bool { return n > 3 })
+// result es Some(4)
+```
+
+## Notas
+
+Funcion pura generica. Implementacion en devfactory/core.