fix: address Final Verification Wave findings

- Sync() now flushes BlockWriter before fd.Sync() for durability
- Pre-validate encoding before sequence allocation (design doc compliance)
- Remove dead _ = rec assignment in recover.go
- Remove unused maxPayload field from SegmentWriter
- Handle Put error in MemTable.Publish with panic on invariant violation

wal/segment_writer.go

@@ -19,7 +19,6 @@ type SegmentWriter struct {
 	startSequence uint64
 	blockWriter   *BlockWriter
 	currentOffset uint64 // total bytes written (starts at WalFileHeaderSize)
-	maxPayload    uint64 // cfg.MaxSegmentSize - WalFileHeaderSize
 }
 
 // NewSegmentWriter creates a new WAL segment file and writes the file header.
@@ -89,8 +88,6 @@ func NewSegmentWriter(
 		dirFD.Close()
 	}
 
-	maxPayload := cfg.MaxSegmentSize - WalFileHeaderSize
-
 	return &SegmentWriter{
 		fd:            fd,
 		dir:           dir,
@@ -99,7 +96,6 @@ func NewSegmentWriter(
 		startSequence: startSequence,
 		blockWriter:   NewBlockWriter(),
 		currentOffset: WalFileHeaderSize,
-		maxPayload:    maxPayload,
 	}, nil
 }
 
@@ -133,6 +129,9 @@ func (sw *SegmentWriter) AppendBatch(encodedBatch []byte) error {
 
 // Sync flushes the segment file to durable storage.
 func (sw *SegmentWriter) Sync() error {
+	if err := sw.blockWriter.Flush(sw.fd); err != nil {
+		return fmt.Errorf("wal: flushing partial block before sync: %w", err)
+	}
 	return sw.fd.Sync()
 }