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/recover.go

@@ -183,7 +183,7 @@ func findValidOffset(filePath string) (int64, error) {
 				break
 			}
 
-			rec, consumed, err := DecodePhysicalRecord(blockData[pos:])
+			_, consumed, err := DecodePhysicalRecord(blockData[pos:])
 			if err != nil {
 				// Corruption starts here — offset is up to last valid record.
 				validOffset = blockStartOffset + int64(pos)
@@ -192,8 +192,6 @@ func findValidOffset(filePath string) (int64, error) {
 
 			// Valid record found.
 			validOffset = blockStartOffset + int64(pos+consumed)
-			// Keep the rec reference alive so the compiler doesn't optimize it away.
-			_ = rec
 			pos += consumed
 		}