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/drivers.go

@@ -0,0 +1,67 @@
+package postcard
+
+import (
+	"context"
+	"fmt"
+	"sync"
+)
+
+// Driver delivers a fully rendered message.
+type Driver interface {
+	Send(ctx context.Context, msg RenderedMessage) error
+}
+
+// RenderedMessage is the validated payload a driver transmits.
+type RenderedMessage struct {
+	To      []string
+	Cc      []string
+	Bcc     []string
+	From    string
+	ReplyTo string
+	Subject string
+	HTML    string
+	Text    string
+}
+
+// MemoryDriver stores rendered messages for tests. It is concurrency-safe.
+type MemoryDriver struct {
+	mu       sync.Mutex
+	messages []RenderedMessage
+}
+
+// NewMemoryDriver returns an empty in-memory driver.
+func NewMemoryDriver() *MemoryDriver {
+	return &MemoryDriver{}
+}
+
+// Send appends msg to the in-memory log.
+func (d *MemoryDriver) Send(_ context.Context, msg RenderedMessage) error {
+	if d == nil {
+		return fmt.Errorf("postcard: memory driver is nil")
+	}
+	d.mu.Lock()
+	defer d.mu.Unlock()
+	d.messages = append(d.messages, cloneRendered(msg))
+	return nil
+}
+
+// Messages returns a copy of stored messages in send order.
+func (d *MemoryDriver) Messages() []RenderedMessage {
+	if d == nil {
+		return nil
+	}
+	d.mu.Lock()
+	defer d.mu.Unlock()
+	out := make([]RenderedMessage, len(d.messages))
+	for i, msg := range d.messages {
+		out[i] = cloneRendered(msg)
+	}
+	return out
+}
+
+func cloneRendered(msg RenderedMessage) RenderedMessage {
+	msg.To = append([]string(nil), msg.To...)
+	msg.Cc = append([]string(nil), msg.Cc...)
+	msg.Bcc = append([]string(nil), msg.Bcc...)
+	return msg
+}