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/pipeline.go

@@ -0,0 +1,13 @@
+package core
+
+// Pipeline composes a sequence of functions T -> T into a single function.
+// The functions are applied left to right: Pipeline(f, g, h)(x) == h(g(f(x))).
+// Returns the identity function if no functions are provided.
+func Pipeline[T any](fns ...func(T) T) func(T) T {
+	return func(val T) T {
+		for _, fn := range fns {
+			val = fn(val)
+		}
+		return val
+	}
+}