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/infra/http_get_json.md

@@ -0,0 +1,43 @@
+---
+name: http_get_json
+kind: function
+lang: go
+domain: infra
+version: "1.0.0"
+purity: impure
+signature: "func HttpGetJSON(url string, headers map[string]string, timeout time.Duration) (map[string]any, error)"
+description: "GET request que espera JSON. Agrega Accept: application/json automaticamente. Retorna error con status code si >= 400. Siempre cierra body con defer."
+tags: [http, json, get, client, network, stdlib, infra]
+uses_functions: []
+uses_types: []
+returns: []
+returns_optional: false
+error_type: "error_go_core"
+imports: ["encoding/json", "fmt", "io", "net/http", "time"]
+tested: true
+tests:
+  - "httptest.Server con respuesta JSON"
+  - "Status 404 → error"
+  - "Timeout → error"
+  - "Headers custom"
+test_file_path: "functions/infra/http_get_json_test.go"
+file_path: "functions/infra/http_get_json.go"
+---
+
+## Ejemplo
+
+```go
+result, err := HttpGetJSON(
+    "https://api.example.com/users",
+    map[string]string{"X-Api-Key": "secret"},
+    10*time.Second,
+)
+if err != nil {
+    return nil, err
+}
+fmt.Println(result["total"])
+```
+
+## Notas
+
+Solo usa stdlib (net/http, encoding/json). El timeout se configura en el http.Client. El error incluye los primeros 200 bytes del body para facilitar debugging. Los headers custom se fusionan con Accept: application/json (custom tiene precedencia).