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.go

@@ -0,0 +1,96 @@
+package datascience
+
+import "fmt"
+
+// DiffEntities compares two snapshots of entities and returns field-level differences.
+// Detects added, removed, modified, and unchanged entities.
+// ignoreFields specifies fields to exclude from comparison (defaults to ["created_at", "updated_at"] when nil).
+func DiffEntities(before, after []map[string]any, key string, ignoreFields []string) map[string]any {
+	if ignoreFields == nil {
+		ignoreFields = []string{"created_at", "updated_at"}
+	}
+
+	ignoreSet := make(map[string]bool, len(ignoreFields))
+	for _, f := range ignoreFields {
+		ignoreSet[f] = true
+	}
+
+	beforeMap := make(map[string]map[string]any, len(before))
+	for _, e := range before {
+		if k, ok := e[key]; ok {
+			beforeMap[fmt.Sprintf("%v", k)] = e
+		}
+	}
+
+	afterMap := make(map[string]map[string]any, len(after))
+	for _, e := range after {
+		if k, ok := e[key]; ok {
+			afterMap[fmt.Sprintf("%v", k)] = e
+		}
+	}
+
+	added := []map[string]any{}
+	for k, e := range afterMap {
+		if _, exists := beforeMap[k]; !exists {
+			added = append(added, e)
+		}
+	}
+
+	removed := []map[string]any{}
+	for k, e := range beforeMap {
+		if _, exists := afterMap[k]; !exists {
+			removed = append(removed, e)
+		}
+	}
+
+	modified := []map[string]any{}
+	unchanged := 0
+
+	for k, b := range beforeMap {
+		a, exists := afterMap[k]
+		if !exists {
+			continue
+		}
+
+		// Collect all fields from both entities
+		allFields := make(map[string]bool)
+		for f := range b {
+			allFields[f] = true
+		}
+		for f := range a {
+			allFields[f] = true
+		}
+
+		changes := map[string]any{}
+		for field := range allFields {
+			if ignoreSet[field] || field == key {
+				continue
+			}
+			oldVal := b[field]
+			newVal := a[field]
+			if fmt.Sprintf("%v", oldVal) != fmt.Sprintf("%v", newVal) {
+				changes[field] = map[string]any{"old": oldVal, "new": newVal}
+			}
+		}
+
+		if len(changes) > 0 {
+			modified = append(modified, map[string]any{"key": k, "changes": changes})
+		} else {
+			unchanged++
+		}
+	}
+
+	nAdded := len(added)
+	nRemoved := len(removed)
+	nModified := len(modified)
+	summary := fmt.Sprintf("%d added, %d removed, %d modified, %d unchanged",
+		nAdded, nRemoved, nModified, unchanged)
+
+	return map[string]any{
+		"added":     added,
+		"removed":   removed,
+		"modified":  modified,
+		"unchanged": unchanged,
+		"summary":   summary,
+	}
+}