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/assets/default.htm

@@ -0,0 +1,14 @@
+name = "Default"
+==
+{{ .Content }}
+==
+<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
+<html xmlns="http://www.w3.org/1999/xhtml">
+<head>
+<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
+<style type="text/css">{{ .brandCss }}{{ .css }}</style>
+</head>
+<body>
+<div class="content-body">{{ .Content }}</div>
+</body>
+</html>