feat: implement working Kafka consumer functionality with stored record batches

- Fixed Produce v2+ handler to properly store messages in ledger and update high water mark
- Added record batch storage system to cache actual Produce record batches
- Modified Fetch handler to return stored record batches instead of synthetic ones
- Consumers can now successfully fetch and decode messages with correct CRC validation
- Sarama consumer successfully consumes messages (1/3 working, investigating offset handling)

Key improvements:
- Produce handler now calls AssignOffsets() and AppendRecord() correctly
- High water mark properly updates from 0 → 1 → 2 → 3
- Record batches stored during Produce and retrieved during Fetch
- CRC validation passes because we return exact same record batch data
- Debug logging shows 'Using stored record batch for offset X'

TODO: Fix consumer offset handling when fetchOffset == highWaterMark

test/kafka/sarama_simple_test.go

@@ -182,7 +182,7 @@ func TestSaramaProduceConsume(t *testing.T) {
 	// Test messages
 	testMessages := []string{
 		"Sarama Producer Message 1",
-		"Sarama Producer Message 2", 
+		"Sarama Producer Message 2",
 		"Sarama Producer Message 3",
 	}