fix(client): don't create local file if remote file doesn't exist

Previously, the get command would create/truncate the local file before
starting the download, then delete it on error. This could inadvertently
destroy an existing local file if the remote file didn't exist.

Now the download completes to memory first, and the local file is only
written after a successful transfer. This is safe for TFTP since files
are typically small (bootloaders, configs, etc.).

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

crates/pfs-tftp/src/bin/tftp.rs

@@ -248,30 +248,28 @@ fn main() -> ExitCode {
                 }
             };
 
-            // Create output file
-            let mut file = match File::create(&local_file) {
-                Ok(f) => f,
+            // Download to memory first - only create local file on success
+            let data = match client.get(&remote_file, args.mode) {
+                Ok(data) => data,
                 Err(e) => {
-                    eprintln!("Error creating file '{local_file}': {e}");
+                    eprintln!("Error: {e}");
                     return ExitCode::FAILURE;
                 }
             };
 
-            // Download
-            match client.get_to_writer(&remote_file, args.mode, &mut file) {
-                Ok(bytes) => {
-                    if args.verbose {
-                        eprintln!("Received {bytes} bytes");
-                    }
-                    println!("Downloaded '{remote_file}' -> '{local_file}' ({bytes} bytes)");
-                }
-                Err(e) => {
-                    // Clean up partial file
-                    let _ = std::fs::remove_file(&local_file);
-                    eprintln!("Error: {e}");
+            // Write to local file
+            if let Err(e) = std::fs::write(&local_file, &data) {
+                eprintln!("Error writing file '{local_file}': {e}");
                 return ExitCode::FAILURE;
             }
+
+            if args.verbose {
+                eprintln!("Received {} bytes", data.len());
             }
+            println!(
+                "Downloaded '{remote_file}' -> '{local_file}' ({} bytes)",
+                data.len()
+            );
         }
 
         Command::Put {