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/chunk.go

@@ -0,0 +1,25 @@
+package core
+
+// Chunk splits xs into sub-slices of the given size.
+// The last chunk may contain fewer than size elements.
+// Returns nil if xs is empty. Panics if size <= 0.
+func Chunk[T any](xs []T, size int) [][]T {
+	if size <= 0 {
+		panic("chunk size must be > 0")
+	}
+	n := len(xs)
+	if n == 0 {
+		return nil
+	}
+
+	numChunks := (n + size - 1) / size
+	chunks := make([][]T, 0, numChunks)
+	for i := 0; i < n; i += size {
+		end := i + size
+		if end > n {
+			end = n
+		}
+		chunks = append(chunks, xs[i:end])
+	}
+	return chunks
+}