fix: add retry with backoff to Databento download_quarter

3 retries with exponential backoff (2s, 4s, 8s) for transient
connection failures (IncompleteBody/UnexpectedEof). Partial files
are deleted before retry so skip-existing logic works correctly.

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

crates/ml/examples/download_baseline.rs

@@ -284,6 +284,9 @@ impl DownloadStats {
     }
 }
 
+/// Maximum retries for transient download failures (connection resets, EOF).
+const MAX_RETRIES: u32 = 3;
+
 async fn download_quarter(
     client: &mut HistoricalClient,
     symbol: &str,
@@ -307,28 +310,51 @@ async fn download_quarter(
 
     let dt_range = date_range(quarter.start, quarter.end)?;
 
-    let params = GetRangeToFileParams::builder()
-        .dataset(dataset.to_owned())
-        .symbols(vec![symbol.to_owned()])
-        .schema(schema)
-        .stype_in(SType::Parent)
-        .date_time_range(dt_range)
-        .path(file_path.clone())
-        .build();
+    let mut last_err = None;
+    for attempt in 0..=MAX_RETRIES {
+        if attempt > 0 {
+            // Remove partial file from previous failed attempt
+            if file_path.exists() {
+                let _ = fs::remove_file(&file_path);
+            }
+            let delay = std::time::Duration::from_secs(2u64.pow(attempt));
+            println!(
+                "  [RETRY] {} attempt {}/{} in {}s",
+                filename,
+                attempt + 1,
+                MAX_RETRIES + 1,
+                delay.as_secs()
+            );
+            tokio::time::sleep(delay).await;
+        }
 
-    let _decoder = client
-        .timeseries()
-        .get_range_to_file(&params)
-        .await
-        .with_context(|| {
-            format!(
-                "Failed to download {} for {}",
-                quarter.label, symbol
-            )
-        })?;
+        let params = GetRangeToFileParams::builder()
+            .dataset(dataset.to_owned())
+            .symbols(vec![symbol.to_owned()])
+            .schema(schema)
+            .stype_in(SType::Parent)
+            .date_time_range(dt_range.clone())
+            .path(file_path.clone())
+            .build();
 
-    let meta = fs::metadata(&file_path).context("Failed to read downloaded file metadata")?;
-    Ok(meta.len())
+        match client.timeseries().get_range_to_file(&params).await {
+            Ok(_decoder) => {
+                let meta =
+                    fs::metadata(&file_path).context("Failed to read downloaded file metadata")?;
+                return Ok(meta.len());
+            }
+            Err(e) => {
+                last_err = Some(anyhow::anyhow!(
+                    "Failed to download {} for {}: {}",
+                    quarter.label,
+                    symbol,
+                    e
+                ));
+            }
+        }
+    }
+
+    Err(last_err.unwrap())
 }
 
 /// Upload a single file to MinIO/S3 via rclone.
@@ -598,7 +624,7 @@ async fn main() -> Result<()> {
                             (false, false, size, quarter.label.clone())
                         }
                         Err(e) => {
-                            println!("  [FAIL] {} -- {}", filename, e);
+                            println!("  [FAIL] {} -- {:?}", filename, e);
                             (false, true, 0, quarter.label.clone())
                         }
                     }