feat(ingest): batch event writes and harden transport

internal/sdk/emitter.go

@@ -221,7 +221,7 @@ func (e *Emitter) Emit(ctx context.Context, event Event) error {
 
 	e.buffer = append(e.buffer, event)
 	if len(e.buffer) >= e.bufferSize {
-		return e.Flush(ctx)
+		return e.flushLocked(ctx)
 	}
 	return nil
 }
@@ -235,6 +235,11 @@ func (e *Emitter) Flush(ctx context.Context) error {
 		return fmt.Errorf("emitter is closed")
 	}
 
+	return e.flushLocked(ctx)
+}
+
+// flushLocked sends buffered events. Caller must hold e.mu.
+func (e *Emitter) flushLocked(ctx context.Context) error {
 	if len(e.buffer) == 0 {
 		return nil
 	}
@@ -281,7 +286,7 @@ func (e *Emitter) Close(ctx context.Context) error {
 	}
 
 	if len(e.buffer) > 0 {
-		_ = e.Flush(ctx)
+		_ = e.flushLocked(ctx)
 	}
 
 	return nil