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/writer.go

@@ -145,6 +145,12 @@ func (ww *WalWriter) processBatch(requests []*CommitRequest) {
 		return
 	}
 
+	// Pre-validate encoding can succeed before sequence allocation.
+	if _, err := EncodeWalBatch(0, entries); err != nil {
+		ww.sendError(requests, err)
+		return
+	}
+
 	baseSequence, err := ww.seqManager.AllocateBatch(uint32(len(entries)))
 	if err != nil {
 		ww.sendError(requests, err)