feat: implement date segment type for part number generation

Fixes #79

Implement the date segment type in the part number generator. Uses Go's
time.Format with the segment's Value field as the layout string.

- Default format: 20060102 (YYYYMMDD) when no Value is specified
- Custom formats via Value field: "0601" (YYMM), "2006" (YYYY), etc.
- Always uses UTC time
- Add 3 tests: default format, custom YYMM format, year-only format

internal/partnum/generator.go

@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"regexp"
 	"strings"
+	"time"
 
 	"github.com/kindredsystems/silo/internal/schema"
 )
@@ -99,8 +100,11 @@ func (g *Generator) resolveSegment(
 		return g.formatSerial(seg, next), nil
 
 	case "date":
-		// TODO: implement date formatting
-		return "", fmt.Errorf("date segments not yet implemented")
+		layout := seg.Value
+		if layout == "" {
+			layout = "20060102"
+		}
+		return time.Now().UTC().Format(layout), nil
 
 	default:
 		return "", fmt.Errorf("unknown segment type: %s", seg.Type)

internal/partnum/generator_test.go

@@ -3,7 +3,9 @@ package partnum
 import (
 	"context"
 	"fmt"
+	"regexp"
 	"testing"
+	"time"
 
 	"github.com/kindredsystems/silo/internal/schema"
 )
@@ -165,3 +167,87 @@ func TestGenerateConstantSegment(t *testing.T) {
 		t.Errorf("got %q, want %q", pn, "KS-0001")
 	}
 }
+
+func TestGenerateDateSegmentDefault(t *testing.T) {
+	s := &schema.Schema{
+		Name:      "date-test",
+		Version:   1,
+		Separator: "-",
+		Segments: []schema.Segment{
+			{Name: "date", Type: "date"},
+			{Name: "serial", Type: "serial", Length: 3},
+		},
+	}
+	gen := NewGenerator(map[string]*schema.Schema{"date-test": s}, &mockSeqStore{})
+
+	pn, err := gen.Generate(context.Background(), Input{
+		SchemaName: "date-test",
+		Values:     map[string]string{},
+	})
+	if err != nil {
+		t.Fatalf("Generate returned error: %v", err)
+	}
+
+	// Default format: YYYYMMDD-NNN
+	want := time.Now().UTC().Format("20060102") + "-001"
+	if pn != want {
+		t.Errorf("got %q, want %q", pn, want)
+	}
+}
+
+func TestGenerateDateSegmentCustomFormat(t *testing.T) {
+	s := &schema.Schema{
+		Name:      "date-custom",
+		Version:   1,
+		Separator: "-",
+		Segments: []schema.Segment{
+			{Name: "date", Type: "date", Value: "0601"},
+			{Name: "serial", Type: "serial", Length: 4},
+		},
+	}
+	gen := NewGenerator(map[string]*schema.Schema{"date-custom": s}, &mockSeqStore{})
+
+	pn, err := gen.Generate(context.Background(), Input{
+		SchemaName: "date-custom",
+		Values:     map[string]string{},
+	})
+	if err != nil {
+		t.Fatalf("Generate returned error: %v", err)
+	}
+
+	// Format "0601" produces YYMM
+	if matched, _ := regexp.MatchString(`^\d{4}-\d{4}$`, pn); !matched {
+		t.Errorf("got %q, want pattern YYMM-NNNN", pn)
+	}
+
+	want := time.Now().UTC().Format("0601") + "-0001"
+	if pn != want {
+		t.Errorf("got %q, want %q", pn, want)
+	}
+}
+
+func TestGenerateDateSegmentYearOnly(t *testing.T) {
+	s := &schema.Schema{
+		Name:      "date-year",
+		Version:   1,
+		Separator: "-",
+		Segments: []schema.Segment{
+			{Name: "year", Type: "date", Value: "2006"},
+			{Name: "serial", Type: "serial", Length: 4},
+		},
+	}
+	gen := NewGenerator(map[string]*schema.Schema{"date-year": s}, &mockSeqStore{})
+
+	pn, err := gen.Generate(context.Background(), Input{
+		SchemaName: "date-year",
+		Values:     map[string]string{},
+	})
+	if err != nil {
+		t.Fatalf("Generate returned error: %v", err)
+	}
+
+	want := time.Now().UTC().Format("2006") + "-0001"
+	if pn != want {
+		t.Errorf("got %q, want %q", pn, want)
+	}
+}