fix: prevent double-escaping of newlines in D2 container labels

escape_d2() was applied after name_lines(), which turned the \n line
separators into \\n (literal backslash-n in D2 output). Swapping the
order escapes D2-special characters first, then inserts line breaks
that D2 interprets correctly.

Author: cds-amal

src/mk_graph/output/d2.rs

@@ -66,7 +66,7 @@ fn render_d2_function(
     out: &mut String,
 ) {
     let fn_id = short_name(name);
-    let display_name = escape_d2(&name_lines(name));
+    let display_name = name_lines(&escape_d2(name));
 
     // Function container
     out.push_str(&format!("{}: {{\n", fn_id));