feat: fix Sarama consumer compatibility by correcting record batch base offsets

🎉 MAJOR SUCCESS: Both kafka-go and Sarama now fully working!

Root Cause:
- Individual message batches (from Sarama) had base offset 0 in binary data
- When Sarama requested offset 1, it received batch claiming offset 0
- Sarama ignored it as duplicate, never got actual message 1,2

Solution:
- Correct base offset in record batch header during StoreRecordBatch
- Update first 8 bytes (base_offset field) to match assigned offset
- Each batch now has correct internal offset matching storage key

Results:
✅ kafka-go: 3/3 produced, 3/3 consumed
✅ Sarama: 3/3 produced, 3/3 consumed

Both clients now have full produce-consume compatibility

weed/mq/kafka/protocol/fetch.go

@@ -101,7 +101,7 @@ func (h *Handler) handleFetch(correlationID uint32, apiVersion uint16, requestBo
 					if len(storedBatch) < hexLen {
 						hexLen = len(storedBatch)
 					}
-					fmt.Printf("DEBUG: Using stored record batch for offset %d, size: %d bytes, first %d bytes: %x\n", 
+					fmt.Printf("DEBUG: Using stored record batch for offset %d, size: %d bytes, first %d bytes: %x\n",
 						partition.FetchOffset, len(storedBatch), hexLen, storedBatch[:hexLen])
 				} else {
 					fmt.Printf("DEBUG: No stored record batch found for offset %d, using synthetic batch\n", partition.FetchOffset)

weed/mq/kafka/protocol/handler.go

@@ -152,9 +152,28 @@ func (h *Handler) GetLedger(topic string, partition int32) *offset.Ledger {
 // StoreRecordBatch stores a record batch for later retrieval during Fetch operations
 func (h *Handler) StoreRecordBatch(topicName string, partition int32, baseOffset int64, recordBatch []byte) {
 	key := fmt.Sprintf("%s:%d:%d", topicName, partition, baseOffset)
-	h.recordBatchMu.Lock()
-	defer h.recordBatchMu.Unlock()
-	h.recordBatches[key] = recordBatch
+	
+	// Fix the base offset in the record batch binary data to match the assigned offset
+	// The base offset is stored in the first 8 bytes of the record batch
+	if len(recordBatch) >= 8 {
+		// Create a copy to avoid modifying the original
+		fixedBatch := make([]byte, len(recordBatch))
+		copy(fixedBatch, recordBatch)
+		
+		// Update the base offset (first 8 bytes, big endian)
+		binary.BigEndian.PutUint64(fixedBatch[0:8], uint64(baseOffset))
+		
+		h.recordBatchMu.Lock()
+		defer h.recordBatchMu.Unlock()
+		h.recordBatches[key] = fixedBatch
+		
+		fmt.Printf("DEBUG: Stored record batch with corrected base offset %d (was %d)\n", 
+			baseOffset, binary.BigEndian.Uint64(recordBatch[0:8]))
+	} else {
+		h.recordBatchMu.Lock()
+		defer h.recordBatchMu.Unlock()
+		h.recordBatches[key] = recordBatch
+	}
 }
 
 // GetRecordBatch retrieves a stored record batch that contains the requested offset
@@ -193,12 +212,12 @@ func (h *Handler) GetRecordBatch(topicName string, partition int32, offset int64
 
 		if recordCount > 0 && offset >= baseOffset && offset < baseOffset+int64(recordCount) {
 			fmt.Printf("DEBUG: Found matching batch for offset %d in batch with baseOffset %d\n", offset, baseOffset)
-			
+
 			// If requesting the base offset, return the entire batch
 			if offset == baseOffset {
 				return batch, true
 			}
-			
+
 			// For non-base offsets, we need to create a sub-batch starting from the requested offset
 			// This is a complex operation, so for now return the entire batch
 			// TODO: Implement proper sub-batch extraction