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/rewrite_rule.go

@@ -0,0 +1,58 @@
+package core
+
+import (
+	"fmt"
+	"regexp"
+	"strings"
+)
+
+var rewriteFieldPattern = regexp.MustCompile(`\b([a-zA-Z_][a-zA-Z0-9_]*)\b`)
+
+var rewriteSQLKeywords = map[string]bool{
+	"AND": true, "OR": true, "NOT": true, "IS": true, "NULL": true,
+	"IN": true, "BETWEEN": true, "LIKE": true, "GLOB": true,
+	"TRUE": true, "FALSE": true, "CASE": true, "WHEN": true,
+	"THEN": true, "ELSE": true, "END": true, "SELECT": true,
+	"FROM": true, "WHERE": true, "AS": true, "CAST": true,
+}
+
+var rewriteSQLFunctions = map[string]bool{
+	"json_extract": true, "datetime": true, "now": true,
+	"abs": true, "avg": true, "count": true, "max": true, "min": true,
+	"sum": true, "total": true, "length": true, "typeof": true,
+	"coalesce": true, "ifnull": true, "nullif": true,
+	"upper": true, "lower": true, "trim": true, "replace": true,
+	"substr": true, "instr": true, "round": true,
+}
+
+// RewriteRule transforms a rule expression so that bare field names become
+// json_extract calls on a given JSON column.
+// For example, with column "metadata":
+//
+//	"price > 100 AND status IS NOT NULL"
+//
+// becomes:
+//
+//	"json_extract(metadata, '$.price') > 100 AND json_extract(metadata, '$.status') IS NOT NULL"
+//
+// If the rule already contains json_extract, it is returned as-is.
+// SQL keywords and common SQL functions are preserved.
+func RewriteRule(rule, jsonColumn string) string {
+	if strings.Contains(rule, "json_extract") {
+		return rule
+	}
+
+	return rewriteFieldPattern.ReplaceAllStringFunc(rule, func(match string) string {
+		upper := strings.ToUpper(match)
+		if rewriteSQLKeywords[upper] {
+			return match
+		}
+		if rewriteSQLFunctions[strings.ToLower(match)] {
+			return match
+		}
+		if match[0] >= '0' && match[0] <= '9' {
+			return match
+		}
+		return fmt.Sprintf("json_extract(%s, '$.%s')", jsonColumn, match)
+	})
+}