feat: funciones Go — core (cron, join_by_key, validate_struct), datascience (pivot, diff_entities), infra (http, cache, cron_ticker)

Nuevas funciones Go con tests en tres dominios:
- core: parse_cron_expr, next_cron_time, join_by_key, validate_struct_fields + tipo CronSchedule
- datascience: pivot (tabla dinámica), diff_entities (comparación de entidades)
- infra: http_get_json, http_post_json, http_download_file, cache_to_sqlite, cron_ticker

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

functions/datascience/diff_entities.md

@@ -0,0 +1,52 @@
+---
+name: diff_entities
+kind: function
+lang: go
+domain: datascience
+version: "1.0.0"
+purity: pure
+signature: "func DiffEntities(before, after []map[string]any, key string, ignoreFields []string) map[string]any"
+description: "Compara dos snapshots de entities y devuelve diferencias campo a campo. Detecta añadidas, eliminadas, modificadas e inalteradas. Ignora created_at y updated_at por defecto (pasar nil para usar defaults)."
+tags: [datascience, diff, entities, operations, snapshot, comparison]
+uses_functions: []
+uses_types: []
+returns: []
+returns_optional: false
+error_type: ""
+imports: ["fmt"]
+tested: true
+tests:
+  - "entity añadida"
+  - "entity eliminada"
+  - "entity modificada con detalle de campos"
+  - "entities identicas → unchanged"
+  - "ignore_fields funciona"
+  - "lista vacia vs lista con datos"
+  - "summary format correcto"
+test_file_path: "functions/datascience/diff_entities_test.go"
+file_path: "functions/datascience/diff_entities.go"
+---
+
+## Ejemplo
+
+```go
+before := []map[string]any{
+    {"id": "1", "name": "Alice", "status": "active"},
+    {"id": "2", "name": "Bob"},
+}
+after := []map[string]any{
+    {"id": "1", "name": "Alice", "status": "inactive"},
+    {"id": "3", "name": "Carol"},
+}
+result := DiffEntities(before, after, "id", nil)
+// result["summary"] = "1 added, 1 removed, 1 modified, 0 unchanged"
+// result["added"] = [{"id": "3", "name": "Carol"}]
+// result["removed"] = [{"id": "2", "name": "Bob"}]
+// result["modified"] = [{"key": "1", "changes": {"status": {"old": "active", "new": "inactive"}}}]
+```
+
+## Notas
+
+Funcion pura. Compara valores con fmt.Sprintf("%v", ...) para manejar tipos heterogeneos en map[string]any.
+ignoreFields nil usa los defaults ["created_at", "updated_at"]. Para no ignorar ningun campo, pasar []string{}.
+Semantica identica a diff_entities_py_datascience, permite comparar resultados entre ejecuciones del mismo pipeline.