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/dag_validate.go

@@ -0,0 +1,83 @@
+package core
+
+import "fmt"
+
+// DagValidate validates a DagDefinition for structural correctness.
+// Checks: steps have name/ID, no duplicate names/IDs, all depends reference
+// existing steps, no dependency cycles. On success, computes topological levels.
+// Returns warnings for steps with both command and script set.
+func DagValidate(dag DagDefinition) DagValidationResult {
+	result := DagValidationResult{Valid: true}
+
+	// Build name/ID sets and check for missing identifiers and duplicates.
+	seen := make(map[string]bool)
+	for i, step := range dag.Steps {
+		ref := stepRef(step)
+		if ref == "" {
+			result.Errors = append(result.Errors,
+				fmt.Sprintf("step[%d]: must have name or id", i))
+			result.Valid = false
+			continue
+		}
+		if seen[ref] {
+			result.Errors = append(result.Errors,
+				fmt.Sprintf("step[%d]: duplicate name/id %q", i, ref))
+			result.Valid = false
+		}
+		seen[ref] = true
+
+		// Warning: command and script both set.
+		if step.Command != "" && step.Script != "" {
+			result.Warnings = append(result.Warnings,
+				fmt.Sprintf("step %q: has both command and script", ref))
+		}
+	}
+
+	if !result.Valid {
+		return result
+	}
+
+	// Check that all depends reference existing steps.
+	for _, step := range dag.Steps {
+		for _, dep := range step.Depends {
+			if !seen[dep] {
+				result.Errors = append(result.Errors,
+					fmt.Sprintf("step %q: depends on unknown step %q", stepRef(step), dep))
+				result.Valid = false
+			}
+		}
+	}
+
+	if !result.Valid {
+		return result
+	}
+
+	// Topological sort with Kahn's — detects cycles and computes levels.
+	levels, err := DagTopoSort(dag.Steps)
+	if err != nil {
+		result.Errors = append(result.Errors, fmt.Sprintf("cycle detected: %v", err))
+		result.Valid = false
+		return result
+	}
+
+	// Convert [][]DagStep to [][]string for the result.
+	strLevels := make([][]string, len(levels))
+	for i, level := range levels {
+		refs := make([]string, len(level))
+		for j, s := range level {
+			refs[j] = stepRef(s)
+		}
+		strLevels[i] = refs
+	}
+	result.Levels = strLevels
+
+	return result
+}
+
+// stepRef returns the canonical reference for a step (ID preferred, then Name).
+func stepRef(s DagStep) string {
+	if s.ID != "" {
+		return s.ID
+	}
+	return s.Name
+}