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>

examples/hello/plugins/base/views/mail/hello-en.htm

@@ -0,0 +1,7 @@
+subject = "Hello {{ .Name }}"
+description = "English hello greeting"
+layout = "default"
+==
+Hello **{{ .Name }}**.
+
+[Site]({{ .URL }})

examples/hello/plugins/base/views/mail/hello.htm

@@ -0,0 +1,7 @@
+subject = "Witaj {{ .Name }}"
+description = "Hello greeting"
+layout = "default"
+==
+Witaj **{{ .Name }}**.
+
+[Strona]({{ .URL }})