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.md

@@ -0,0 +1,49 @@
+---
+name: cron_match
+kind: function
+lang: go
+domain: core
+version: "1.0.0"
+purity: pure
+signature: "func CronMatch(sched CronSchedule, t time.Time) bool"
+description: "Verifica si un instante de tiempo coincide con un cron schedule. Compara los 5 campos (minuto, hora, dia del mes, mes, dia de la semana) y retorna true si todos coinciden."
+tags: [cron, scheduling, matching, time, pure]
+uses_functions: []
+uses_types: [cron_schedule_go_core]
+returns: []
+returns_optional: false
+error_type: ""
+imports: [time]
+params:
+  - name: sched
+    desc: "CronSchedule con listas de valores validos por campo (resultado de ParseCronExpr)"
+  - name: t
+    desc: "instante de tiempo a verificar contra el schedule"
+output: "true si t coincide con todos los campos del cron schedule"
+tested: true
+tests:
+  - "9:00 AM coincide con 0 9 * * *"
+  - "9:15 AM NO coincide con 0 9 * * *"
+  - "lunes a las 9 coincide con 0 9 * * 1"
+  - "domingo a las 9 NO coincide con 0 9 * * 1"
+  - "wildcard * coincide con cualquier valor"
+  - "specific month"
+test_file_path: "functions/core/cron_match_test.go"
+file_path: "functions/core/cron_match.go"
+---
+
+## Ejemplo
+
+```go
+sched, _ := ParseCronExpr("0 9 * * *")
+t := time.Date(2026, 4, 11, 9, 0, 0, 0, time.UTC)
+CronMatch(sched, t) // true
+
+t2 := time.Date(2026, 4, 11, 10, 0, 0, 0, time.UTC)
+CronMatch(sched, t2) // false
+```
+
+## Notas
+
+Funcion pura. Usa AND semantics para day_of_month y day_of_week (ambos deben coincidir), igual que NextCronTime en el mismo paquete.
+Reutiliza el helper intIn definido en next_cron_time.go (mismo paquete core).