feat: add DAG core functions — parse, validate, topo sort, resolve env, cron match (0007a, 0007d)

Pure functions for parsing dagu-compatible YAML, validating DAG structure,
topological sorting with parallel levels (Kahn's algorithm), and env variable
resolution. Also adds cron_match for schedule matching.

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

functions/core/cron_match.go

@@ -0,0 +1,12 @@
+package core
+
+import "time"
+
+// CronMatch returns true if time t matches all fields of the cron schedule.
+func CronMatch(sched CronSchedule, t time.Time) bool {
+	return intIn(t.Minute(), sched.Minute) &&
+		intIn(t.Hour(), sched.Hour) &&
+		intIn(t.Day(), sched.DayOfMonth) &&
+		intIn(int(t.Month()), sched.Month) &&
+		intIn(int(t.Weekday()), sched.DayOfWeek)
+}