feat: search for existing base VHD in same directory

When only a delta .app is provided without the base, fsdecrypt now
scans the same directory for an existing base VHD matching the
game ID pattern ({game_id}_*_0.vhd) before giving up.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/main.rs

@@ -406,12 +406,48 @@ fn main() -> Result<()> {
                 continue;
             }
 
-            let Some(base) = base else {
-                println!(
-                    "WARNING: Delta VHD(s) found for {game_id} but no base (seq=0) VHD was extracted."
-                );
-                println!("         Provide the base .app file to enable automatic merging.");
-                continue;
+            // If no base was extracted in this run, search the same directory for an existing one
+            let found_base_path;
+            let base = match base {
+                Some(b) => b,
+                None => {
+                    // Look for a VHD matching "{game_id}_*_0.vhd" in the same directory as the delta
+                    let delta_dir = deltas[0]
+                        .vhd_path
+                        .parent()
+                        .unwrap_or_else(|| Path::new("."));
+                    let pattern_prefix = format!("{game_id}_");
+                    let pattern_suffix = "_0.vhd";
+
+                    found_base_path = std::fs::read_dir(delta_dir)?
+                        .filter_map(|e| e.ok())
+                        .map(|e| e.path())
+                        .find(|p| {
+                            let name = p.file_name().unwrap_or_default().to_string_lossy();
+                            name.starts_with(&pattern_prefix) && name.ends_with(pattern_suffix)
+                        });
+
+                    match &found_base_path {
+                        Some(path) => {
+                            println!("Found existing base VHD: {}", path.display());
+                            // Create a temporary ExtractedVhd to reference
+                            &ExtractedVhd {
+                                vhd_path: path.clone(),
+                                sequence_number: 0,
+                                game_id: game_id.clone(),
+                            }
+                        }
+                        None => {
+                            println!(
+                                "WARNING: Delta VHD(s) found for {game_id} but no base (seq=0) VHD found."
+                            );
+                            println!(
+                                "         Provide the base .app file or place the base VHD in the same directory."
+                            );
+                            continue;
+                        }
+                    }
+                }
             };
 
             for delta in deltas {