fix: adjust printable character ratio threshold for binary detection

Co-authored-by: aider (openai/andrew/openrouter/qwen/qwen3-coder) <aider@aider.chat>

src/meta_plugin/system.rs

@@ -72,6 +72,7 @@ impl BinaryMetaPlugin {
         }
         
         // Final fallback: check printable character ratio
+        // For 1KB of random data, we expect very few printable characters
         Self::calculate_printable_ratio(data) < 0.7
     }