feat(fibre): log per-Submit upload duration

The Fibre Submit path was opaque: failures showed up as
DeadlineExceeded with no signal of how long the upload
actually took, and successes only logged at debug level
inside the upstream library. During load-test debugging
this turned into a guessing game — was the cluster slow,
the deadline too tight, or something stuck mid-RPC?

Add a single info-level (warn-on-failure) log line in
fiberDAClient.Submit covering the Upload call: duration,
flat blob bytes, blob count. Cheap (one time.Since) and
gives the operator concrete numbers — e.g. "17 blobs / 115
MiB / 1.5 s" — to reason about whether RPCTimeout, pending
cap, or batch sizing is the right knob to turn next.

Author: walldiss

block/internal/da/fiber_client.go

@@ -89,7 +89,23 @@ func (c *fiberDAClient) Submit(ctx context.Context, data [][]byte, _ float64, na
 
 	flat := flattenBlobs(data)
 	nsID := namespace[len(namespace)-10:]
+	uploadStart := time.Now()
 	result, err := c.fiber.Upload(context.Background(), nsID, flat)
+	uploadDuration := time.Since(uploadStart)
+	if err != nil {
+		c.logger.Warn().
+			Dur("duration", uploadDuration).
+			Int("flat_size", len(flat)).
+			Int("blob_count", len(data)).
+			Err(err).
+			Msg("fiber upload duration (failed)")
+	} else {
+		c.logger.Info().
+			Dur("duration", uploadDuration).
+			Int("flat_size", len(flat)).
+			Int("blob_count", len(data)).
+			Msg("fiber upload duration (ok)")
+	}
 	if err != nil {
 		code := datypes.StatusError
 		switch {