feat: 6 funciones core — retry, memoize, pipeline, map_concurrent, partition, chunk

Funciones genericas reutilizables:
- RetryWithBackoff: reintento con backoff exponencial (impure)
- Memoize: cache de funciones puras (pure)
- Pipeline: composición T→T en secuencia (pure)
- MapConcurrent: map paralelo con worker pool (impure)
- Partition: divide slice en dos por predicado (pure)
- Chunk: divide slice en trozos de tamaño N (pure)
Todas con implementación real y documentación .md.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

functions/core/partition.go

@@ -0,0 +1,16 @@
+package core
+
+// Partition splits xs into two slices: the first contains elements where pred returns true,
+// the second contains elements where pred returns false. Original order is preserved in both.
+func Partition[T any](xs []T, pred func(T) bool) ([]T, []T) {
+	trueSlice := make([]T, 0)
+	falseSlice := make([]T, 0)
+	for _, x := range xs {
+		if pred(x) {
+			trueSlice = append(trueSlice, x)
+		} else {
+			falseSlice = append(falseSlice, x)
+		}
+	}
+	return trueSlice, falseSlice
+}