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/pivot.md

@@ -0,0 +1,43 @@
+---
+name: pivot
+kind: function
+lang: go
+domain: datascience
+version: "1.0.0"
+purity: pure
+signature: "func Pivot(rows []map[string]any, index, columns, values, agg string) []map[string]any"
+description: "Pivot table sin dependencias. Agrupa por index, expande valores unicos de columns como nuevas columnas y agrega values con la funcion indicada (sum, count, mean, min, max, first, last). Valores faltantes se rellenan con 0."
+tags: [datascience, tabular, pivot, transform, aggregation, go]
+uses_functions: []
+uses_types: []
+returns: []
+returns_optional: false
+error_type: ""
+imports: []
+tested: true
+tests:
+  - "Pivot basico con sum"
+  - "Pivot con count y mean"
+  - "Valores faltantes rellenados con 0"
+  - "Una sola fila"
+  - "Multiples valores por celda requieren agregacion"
+test_file_path: "functions/datascience/pivot_test.go"
+file_path: "functions/datascience/pivot.go"
+---
+
+## Ejemplo
+
+```go
+rows := []map[string]any{
+    {"region": "US", "product": "A", "sales": 10},
+    {"region": "US", "product": "B", "sales": 20},
+    {"region": "EU", "product": "A", "sales": 15},
+}
+result := Pivot(rows, "region", "product", "sales", "sum")
+// [{"region": "US", "A": 10.0, "B": 20.0}, {"region": "EU", "A": 15.0, "B": 0}]
+```
+
+## Notas
+
+Funcion pura sin dependencias externas. Usa map[string]any para trabajar con datos JSON/SQL deserializados.
+Las agregaciones numericas (sum, mean, min, max) convierten valores a float64 via type assertion.