feat(04-03): render registered mail templates through memory

- Parse Winter INI + == Markdown templates and Goldmark HTML
- Memory driver stores rendered subject, text, and HTML
- Caller selects -en siblings by full dotted name

Co-authored-by: Cursor <cursoragent@cursor.com>

postcard/mailer.go

@@ -0,0 +1,72 @@
+package postcard
+
+import (
+	"context"
+	"fmt"
+)
+
+// Message is the Send contract for plugin callers.
+type Message struct {
+	Template string
+	To       []string
+	Cc       []string
+	Bcc      []string
+	ReplyTo  string
+	Vars     map[string]any
+	Subject  string
+}
+
+// Mailer sends a registered template through the configured driver.
+type Mailer interface {
+	Send(ctx context.Context, msg Message) error
+}
+
+// Options configure an app-scoped mailer.
+type Options struct {
+	From     string
+	CSS      string
+	BrandCSS string
+}
+
+type mailer struct {
+	catalog *Catalog
+	driver  Driver
+	opts    Options
+}
+
+// NewMailer returns a mailer that renders from cat and delivers through driver.
+func NewMailer(cat *Catalog, driver Driver, opts Options) Mailer {
+	if cat == nil {
+		cat = NewCatalog()
+	}
+	return &mailer{catalog: cat, driver: driver, opts: opts}
+}
+
+// Send renders msg.Template and delivers it. It performs no locale selection;
+// the caller must pass the full dotted name, including any -en suffix.
+func (m *mailer) Send(ctx context.Context, msg Message) error {
+	if m == nil {
+		return fmt.Errorf("postcard: mailer is nil")
+	}
+	if m.driver == nil {
+		return fmt.Errorf("postcard: driver is nil")
+	}
+	if msg.Template == "" {
+		return fmt.Errorf("postcard: template is empty")
+	}
+	if len(msg.To) == 0 && len(msg.Cc) == 0 && len(msg.Bcc) == 0 {
+		return fmt.Errorf("postcard: no recipients")
+	}
+	rendered, err := m.catalog.render(msg, renderOptions{
+		css:      m.opts.CSS,
+		brandCSS: m.opts.BrandCSS,
+	})
+	if err != nil {
+		return err
+	}
+	rendered.From = m.opts.From
+	if err := m.driver.Send(ctx, rendered); err != nil {
+		return fmt.Errorf("postcard: %w", err)
+	}
+	return nil
+}