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

memtable/memtable.go

@@ -2,6 +2,7 @@ package memtable
 
 import (
 	"errors"
+	"fmt"
 	"sync"
 	"sync/atomic"
 )
@@ -114,7 +115,11 @@ func (mt *MemTable) Publish(upToSequence uint64) {
 
 	// Re-put each entry as published. Each call acquires the skiplist mutex.
 	for _, e := range toPublish {
-		_ = mt.skiplist.Put(e.key, e.value, e.sequence, false)
+		if err := mt.skiplist.Put(e.key, e.value, e.sequence, false); err != nil {
+			// Reserve guaranteed space, so this should never happen.
+			// If it does, it's an internal invariant violation.
+			panic(fmt.Sprintf("memtable: Put failed after Reserve: %v", err))
+		}
 	}
 
 	// Update high-water mark after all entries are visible.