fix: 3-level NTX correctness + CDX SET INDEX TO string quoting

NTX 3-level tree (build.go):
- Hybrid approach: bulk build for ≤2 levels, insertKeyBTree for 3+
- rebuildWithInsert: creates proper B-tree via per-key insertion
- 5000-key test: Count=5000 Found=5000 (was 5004/4868)

CDX SET INDEX TO (gengo.go):
- Strip surrounding quotes from string literal in OrderListAdd
- Was: idx.OrderListAdd("\"path\"") → file not found
- Now: idx.OrderListAdd("path") → correct

All tests:
- 14 packages ALL PASS
- 82/82 NTX stress test
- 18/18 CDX cross-read
- 50K benchmark: all counts correct

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

hbrdd/ntx/build.go

@@ -32,8 +32,15 @@ func CreateIndex(path string, keyExpr string, keyLen int, unique bool, descend b
 	maxItem := calculateMaxItems(itemSize)
 	halfPage := maxItem / 2
 
+	// Determine tree depth: if 3+ levels needed, use per-key insertion
+	// for correct B-tree structure. Bulk build only for ≤2 levels.
+	nLeafPages := (len(keys) + maxItem - 1) / maxItem
+	if nLeafPages == 0 {
+		nLeafPages = 1
+	}
+	use3LevelFallback := nLeafPages > maxItem+1
+
 	// Bulk build: all pages in memory buffer, single write at end.
-	// Ported from rddfive/ntx_engine.c fa_ntx_create().
 	var buf pageBuffer
 	buf.init()
 
@@ -166,9 +173,63 @@ func CreateIndex(path string, keyExpr string, keyLen int, unique bool, descend b
 	}
 
 	f.Close()
+
+	if use3LevelFallback && len(keys) > 0 {
+		// 3+ level tree: rebuild using per-key insertion for correct B-tree
+		return rebuildWithInsert(path, keyExpr, keyLen, unique, descend, keys)
+	}
+
 	return OpenIndex(path)
 }
 
+// rebuildWithInsert creates an NTX using per-key insertion (proper B-tree).
+// Used for 3+ level trees where bulk build has separator duplication issues.
+func rebuildWithInsert(path, keyExpr string, keyLen int, unique, descend bool, keys []KeyRecord) (*Index, error) {
+	itemSize := 8 + keyLen
+	maxItem := calculateMaxItems(itemSize)
+	halfPage := maxItem / 2
+
+	f, err := os.Create(path)
+	if err != nil {
+		return nil, err
+	}
+
+	rootOff := int64(HeaderSize)
+	emptyRoot := [BlockSize]byte{}
+	initPageOffsets(emptyRoot[:], maxItem, itemSize)
+
+	hdr := Header{
+		Type: 0x0401, Version: 1,
+		Root: uint32(rootOff), NextPage: uint32(rootOff + BlockSize),
+		ItemSize: uint16(itemSize), KeySize: uint16(keyLen),
+		MaxItem: uint16(maxItem), HalfPage: uint16(halfPage),
+	}
+	copy(hdr.KeyExpr[:], keyExpr)
+	if unique {
+		hdr.Unique = 1
+	}
+	if descend {
+		hdr.Descend = 1
+	}
+	WriteHeader(f, &hdr)
+	f.WriteAt(emptyRoot[:], rootOff)
+	f.Close()
+
+	idx, err := OpenIndex(path)
+	if err != nil {
+		return nil, err
+	}
+	for _, kr := range keys {
+		k := make([]byte, keyLen)
+		copy(k, kr.Key)
+		if err := idx.insertKeyBTree(k, kr.RecNo); err != nil {
+			idx.Close()
+			return nil, err
+		}
+	}
+	return idx, nil
+}
+
 // --- Bulk build buffer (ported from rddfive/ntx_engine.c) ---
 
 type pageBuffer struct {