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_post_json.md

@@ -0,0 +1,43 @@
+---
+name: http_post_json
+kind: function
+lang: go
+domain: infra
+version: "1.0.0"
+purity: impure
+signature: "func HttpPostJSON(url string, body any, headers map[string]string, timeout time.Duration) (map[string]any, error)"
+description: "POST request con body JSON serializado con json.Marshal. Agrega Content-Type: application/json y Accept: application/json. Retorna error con status code si >= 400."
+tags: [http, json, post, client, network, stdlib, infra]
+uses_functions: []
+uses_types: []
+returns: []
+returns_optional: false
+error_type: "error_go_core"
+imports: ["bytes", "encoding/json", "fmt", "io", "net/http", "time"]
+tested: true
+tests:
+  - "httptest.Server recibe body correcto"
+  - "Status 201 → exito"
+  - "Status 500 → error con body parcial"
+test_file_path: "functions/infra/http_post_json_test.go"
+file_path: "functions/infra/http_post_json.go"
+---
+
+## Ejemplo
+
+```go
+result, err := HttpPostJSON(
+    "https://api.example.com/users",
+    map[string]any{"name": "Alice", "role": "admin"},
+    map[string]string{"X-Api-Key": "secret"},
+    10*time.Second,
+)
+if err != nil {
+    return nil, err
+}
+fmt.Println(result["id"])
+```
+
+## Notas
+
+Solo usa stdlib. El body acepta `any` y se serializa con json.Marshal. Headers custom se fusionan con Content-Type y Accept por defecto. El error incluye los primeros 200 bytes del body de respuesta.