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/core/cron_schedule.go

@@ -0,0 +1,11 @@
+package core
+
+// CronSchedule represents a parsed cron expression with expanded field values.
+type CronSchedule struct {
+	Minute     []int
+	Hour       []int
+	DayOfMonth []int
+	Month      []int
+	DayOfWeek  []int
+	Raw        string // original expression
+}