fix: always wrap Big O fallback in O(...) for consistent output

When asymptotic_normal_form fails (e.g., variable^variable like
num_vars^num_vars), the fallback now wraps the raw expression in O(...)
instead of displaying it bare. This ensures consistent format for both
human and JSON output.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: GiggleLiu

problemreductions-cli/src/commands/graph.rs

@@ -242,11 +242,11 @@ pub fn show(problem: &str, out: &OutputConfig) -> Result<()> {
 }
 
 /// Format an expression as Big O notation using asymptotic normalization.
-/// Falls back to the original expression string if normalization fails.
+/// Falls back to wrapping the original expression if normalization fails.
 fn big_o_of(expr: &Expr) -> String {
     match asymptotic_normal_form(expr) {
         Ok(norm) => format!("O({})", norm),
-        Err(_) => expr.to_string(),
+        Err(_) => format!("O({})", expr),
     }
 }