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/memoize.go

@@ -0,0 +1,16 @@
+package core
+
+// Memoize wraps a pure function fn so that results are cached by key.
+// Subsequent calls with the same key return the cached value without calling fn again.
+// The returned function is safe for single-goroutine use.
+func Memoize[K comparable, V any](fn func(K) V) func(K) V {
+	cache := make(map[K]V)
+	return func(key K) V {
+		if val, ok := cache[key]; ok {
+			return val
+		}
+		val := fn(key)
+		cache[key] = val
+		return val
+	}
+}