fix(blockassembly): log columnar->row fallback at debug level

A rolling-deploy mismatch where one block-assembly replica predates bsv-blockchain#889
silently drops to the row path; without a log line operators
investigating "why is this batch slow" can't tell the fallback fired.
Debug level keeps it out of normal log volume but available when
needed.

Author: icellan

services/blockassembly/Client.go

@@ -466,6 +466,7 @@ func (s *Client) sendBatchColumnar(ctx context.Context, batch []*batchItem) {
 		// independently. Costs one wasted RPC per batch against a
 		// persistently-old server; acceptable for the simpler code path.
 		if status.Code(err) == codes.Unimplemented {
+			s.logger.Debugf("[blockassembly] columnar AddTxBatch unimplemented on peer; falling back to row-oriented batch (rolling deploy?): %v", err)
 			s.sendBatchRowOriented(ctx, batch)
 			return
 		}