fix(btree): resolve all 8 Valgrind memcheck defects in BTreeTest

Valgrind (ctest -T memcheck) reported 8 error contexts in BTreeTest.
Root causes, confirmed by reproducing locally under Valgrind:

- B_TREE_CREATE never initialized currentLoadedPages, so
  CHECK_AND_RELEASE_PAGES branched on stack garbage (3 contexts of
  "conditional jump depends on uninitialised value")
- B_TREE_PAGE_CREATE malloc'd pages but left rows[], references[] and
  padding uninitialized while DISK_WRITE persists the whole struct
  (1 context of "syscall write(buf) points to uninitialised bytes");
  now uses calloc. The children-clearing loop also wrote one element
  past the array (the known BTree.c:276 off-by-one flagged by UBSan)
- BTreeTest never freed the tree: TearDown was commented out
  (4 "definitely lost" contexts, up to 2.5 MB in AddMultipleItems);
  B_TREE_FREE verified sound under Valgrind before re-enabling it
- Test rows are now zero-initialized since B_TREE_INSERT persists the
  full row (value union + padding) to disk

Verified with the real GTest suite built against googletest 1.12.1:
4/4 tests pass and Valgrind reports 0 errors from 0 contexts with the
CI memcheck flags and valgrind.supp.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: hbgit

modules/backend/library/lib/BTree.c

@@ -130,6 +130,7 @@ B_TREE_ROW *B_TREE_SEARCH(B_TREE *btree, unsigned key) {
 B_TREE B_TREE_CREATE(const char *filename) {
   B_TREE bt;
   strncpy(bt.filename, filename, FUNCTION_MAX_LENGTH_NAME);
+  bt.currentLoadedPages = 0;
   bt.root = B_TREE_PAGE_CREATE(&bt);
   bt.root->isLeaf = TRUE;
   return bt;
@@ -261,7 +262,9 @@ Bool B_TREE_SPLIT_CHILD(B_TREE *btree, B_TREE_PAGE *X, int index,
 
 B_TREE_PAGE *B_TREE_PAGE_CREATE(B_TREE *btree) {
   btree->currentLoadedPages += 1;
-  B_TREE_PAGE *btp = malloc(sizeof(B_TREE_PAGE));
+  /* calloc: the whole page (rows, references, padding) is written to disk
+   * by DISK_WRITE, so every byte must be initialized */
+  B_TREE_PAGE *btp = calloc(1, sizeof(B_TREE_PAGE));
   if (btp == NULL) {
     return NULL;
   }
@@ -271,12 +274,9 @@ B_TREE_PAGE *B_TREE_PAGE_CREATE(B_TREE *btree) {
   btp->isLeaf = TRUE;
 
   int i = 0;
-  for (; i < B_TREE_MAP2CHECK_ORDER * 2 + 1; i++) {
-    // btp->rows[i] = NULL;
+  for (; i < B_TREE_MAP2CHECK_ORDER * 2; i++) {
     btp->children[i] = NULL;
-    // btp->references[i] = -1;
   }
-  // btp->references[i] = -1;
 
   if (!DISK_WRITE(btree, btp)) {
     return NULL;

tests/unit/backend-library/BTreeTest.cpp

@@ -9,7 +9,7 @@ class BTreeTest : public ::testing::Test {
     system("rm -rf btree_test.bin");
     bt = B_TREE_CREATE("btree_test.bin");
   }
-  //   void TearDown() override { B_TREE_FREE(&bt); }
+  void TearDown() override { B_TREE_FREE(&bt); }
   B_TREE bt;
 };
 
@@ -18,13 +18,15 @@ TEST_F(BTreeTest, ConstructorShouldBeSettedRight) {
 }
 
 TEST_F(BTreeTest, AddItem) {
-  B_TREE_ROW row;
+  // Zero-initialized: the whole row (value union + padding) is persisted
+  // to disk by B_TREE_INSERT, so every byte must be defined.
+  B_TREE_ROW row = {};
   row.index = 0;
   B_TREE_INSERT(&bt, &row);
 }
 
 TEST_F(BTreeTest, AddItemShouldSetValue) {
-  B_TREE_ROW row;
+  B_TREE_ROW row = {};
   row.index = 0;
   B_TREE_INSERT(&bt, &row);
 
@@ -35,8 +37,8 @@ TEST_F(BTreeTest, AddItemShouldSetValue) {
 }
 
 TEST_F(BTreeTest, AddMultipleItems) {
-  unsigned length = 10000;
-  B_TREE_ROW row[length];
+  const unsigned length = 10000;
+  B_TREE_ROW row[length] = {};
   for (int i = 0; i < length; i++) {
     row[i].index = i + 1;
     ;