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/partial2.md

@@ -0,0 +1,33 @@
+---
+name: partial2
+kind: function
+lang: go
+domain: core
+version: "1.0.0"
+purity: pure
+signature: "func Partial2[A, B, C any](f func(A, B) C, a A) func(B) C"
+description: "Aplica parcialmente el primer argumento de una funcion de dos parametros."
+tags: [functional, generic, partial]
+uses_functions: []
+uses_types: []
+returns: []
+returns_optional: false
+error_type: ""
+imports: []
+tested: false
+tests: []
+test_file_path: ""
+file_path: "functions/core/partial2.go"
+---
+
+## Ejemplo
+
+```go
+mul := func(a, b int) int { return a * b }
+double := Partial2(mul, 2)
+result := double(5) // 10
+```
+
+## Notas
+
+Funcion pura generica. Implementacion en devfactory/core.