Use write_all for ledger entries to improve concurrent-write safety

writeln!(f, ...) makes two syscalls (data + newline) which can interleave
between concurrent processes even with O_APPEND. Serialise entry to bytes
and append the newline before write_all() so the entire entry lands in a
single write() syscall, which O_APPEND makes atomic on Linux local
filesystems for typical entry sizes.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/agent.rs

@@ -617,18 +617,22 @@ fn append_ledger_entry(ledger: &Path, result: &BacktestResult, strategy: &Value)
             .to_string(),
         strategy: strategy.clone(),
     };
-    let line = match serde_json::to_string(&entry) {
-        Ok(s) => s,
+    // Append newline inside the serialised bytes so the entire write is a single
+    // write_all() syscall — O_APPEND + single write() is atomic on Linux local
+    // filesystems, making concurrent instances safe for typical entry sizes.
+    let mut bytes = match serde_json::to_vec(&entry) {
+        Ok(b) => b,
         Err(e) => {
             warn!("could not serialize ledger entry: {e}");
             return;
         }
     };
+    bytes.push(b'\n');
     if let Err(e) = std::fs::OpenOptions::new()
         .append(true)
         .create(true)
         .open(ledger)
-        .and_then(|mut f| writeln!(f, "{}", line))
+        .and_then(|mut f| f.write_all(&bytes))
     {
         warn!("could not write ledger entry: {e}");
     }