feat: Add is_internal method to CompressionEngine trait

Co-authored-by: aider (openai/andrew/openrouter/sonoma-sky-alpha) <aider@aider.chat>

src/compression_engine.rs

@@ -110,6 +110,18 @@ pub trait CompressionEngine {
         true
     }
 
+    /// Checks if this compression engine is internal (built-in) or external (program-based).
+    ///
+    /// Internal engines use Rust implementations without external dependencies.
+    /// External engines rely on system programs.
+    ///
+    /// # Returns
+    ///
+    /// * `bool` - True if internal, false if external.
+    fn is_internal(&self) -> bool {
+        true
+    }
+
     /// Gets status information for this compression engine.
     ///
     /// Provides details about the binary/program used and compression/decompression commands.