fix(faces/pseudo): preserve indentation in transform_set

`transform_set` was calling `String.trim line` immediately, discarding
any leading whitespace. Statements like `    set x to expr` inside a
function body would lower to `let x = expr` at column 0, producing a
snapshot mismatch with the 4-space-indented expected output.

Fix: capture the leading indent before trimming and prepend it to the
generated `let`/`let mut` expression.

https://claude.ai/code/session_01Vrh2f1G8tf7ZbNcKh9r5U9

Author: claude

lib/pseudocode_face.ml

@@ -161,15 +161,17 @@ let transform_function_decl trimmed =
 
 (** [set x to expr] → [let x = expr] *)
 let transform_set line =
-  let line = String.trim line in
-  if starts_with line "set " then begin
-    let rest = String.sub line 4 (String.length line - 4) in
+  let trimmed = String.trim line in
+  let indent_len = String.length line - String.length trimmed in
+  let indent = String.sub line 0 indent_len in
+  if starts_with trimmed "set " then begin
+    let rest = String.sub trimmed 4 (String.length trimmed - 4) in
     (* Look for " to " *)
     match String.split_on_char ' ' rest with
     | name :: "to" :: "mut" :: value_parts ->
-      Printf.sprintf "let mut %s = %s" name (String.concat " " value_parts)
+      indent ^ Printf.sprintf "let mut %s = %s" name (String.concat " " value_parts)
     | name :: "to" :: value_parts ->
-      Printf.sprintf "let %s = %s" name (String.concat " " value_parts)
+      indent ^ Printf.sprintf "let %s = %s" name (String.concat " " value_parts)
     | _ -> line
   end else line