feat: funciones core — detect_cycle, generate_id, rewrite_rule

Tres funciones puras para el dominio core: detección de ciclos en grafos
dirigidos (DFS), generación de IDs determinísticos, y reescritura de
reglas con pattern matching.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

functions/core/detect_cycle.go

@@ -0,0 +1,68 @@
+package core
+
+import (
+	"database/sql"
+	"fmt"
+)
+
+// DetectCycle checks if adding a directed edge (from -> to) would create a cycle
+// in a directed graph stored in a SQLite table.
+// It performs BFS from toNode following edges where the filter column is non-empty.
+// If it reaches fromNode, a cycle exists.
+//
+// Parameters:
+//   - conn: open *sql.DB connection
+//   - table: table name containing the edges (e.g. "relations")
+//   - fromCol: column name for edge source (e.g. "from_entity")
+//   - toCol: column name for edge destination (e.g. "to_entity")
+//   - filterCol: column name that must be non-empty for causal edges (e.g. "via"); pass "" to consider all edges
+//   - fromNode: source node of the proposed new edge
+//   - toNode: destination node of the proposed new edge
+func DetectCycle(conn *sql.DB, table, fromCol, toCol, filterCol, fromNode, toNode string) error {
+	if fromNode == "" || toNode == "" {
+		return nil
+	}
+
+	var query string
+	if filterCol != "" {
+		query = fmt.Sprintf("SELECT %s FROM %s WHERE %s = ? AND %s != ''", toCol, table, fromCol, filterCol)
+	} else {
+		query = fmt.Sprintf("SELECT %s FROM %s WHERE %s = ?", toCol, table, fromCol)
+	}
+
+	visited := map[string]bool{}
+	queue := []string{toNode}
+
+	for len(queue) > 0 {
+		current := queue[0]
+		queue = queue[1:]
+
+		if visited[current] {
+			continue
+		}
+		visited[current] = true
+
+		if current == fromNode {
+			return fmt.Errorf("cycle detected: adding edge %s -> %s would create a cycle", fromNode, toNode)
+		}
+
+		rows, err := conn.Query(query, current)
+		if err != nil {
+			return fmt.Errorf("querying %s for cycle detection: %w", table, err)
+		}
+
+		for rows.Next() {
+			var next string
+			if err := rows.Scan(&next); err != nil {
+				rows.Close()
+				return err
+			}
+			if !visited[next] {
+				queue = append(queue, next)
+			}
+		}
+		rows.Close()
+	}
+
+	return nil
+}