fix(lucid-face): strip extra trailing newline from transform_source

When source ends with \n, String.split_on_char '\n' produces a trailing
empty string. The loop processes it as an empty line, emitting an extra
\n via Buffer.add_char — making output end with \n\n instead of \n.
Strip the redundant newline at the end of transform_source so the output
matches the snapshot (which was already updated to have exactly one
trailing newline).

https://claude.ai/code/session_01Vrh2f1G8tf7ZbNcKh9r5U9

Author: claude

lib/lucid_face.ml

@@ -658,7 +658,10 @@ let transform_source source =
   for _ = 1 to !toplevel_braces do
     Buffer.add_string out "}\n"
   done;
-  Buffer.contents out
+  let s = Buffer.contents out in
+  let l = String.length s in
+  if l >= 2 && s.[l-1] = '\n' && s.[l-2] = '\n' then String.sub s 0 (l-1)
+  else s
 
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