Replace SIMD assembly with initialized Node16::index + valgrind client request

Commit 6fed133 moved SIMD operations on Node16::index into file-level
assembly (later extended to aarch64) to avoid reading indeterminate bytes
(UB in C++). Revert that in favor of a simpler, architecture-independent
fix:

- Restore the original HAS_AVX / HAS_ARM_NEON intrinsics at the affected
  call sites, keeping the newer conflictMask16 helper and scalar fallback.
- memset Node16::index in both copy constructors (from Node3 and Node48) so
  the [numChildren, 16) tail is always initialized, making the full 16-byte
  SIMD loads well-defined. This also fixes the same latent UB in the NEON
  path and the Node16->Node16 full-array memcpy.
- Issue a VALGRIND_MAKE_MEM_UNDEFINED client request on the tail so valgrind
  still flags any read that relies on the unused slots rather than numChildren.
- Drop simd.h, simd_x86_64.S, simd_aarch64.S, and the CMake ASM wiring.

ConflictSet.cpp

@@ -549,6 +549,7 @@ inline void Node48::copyChildrenAndKeyFrom(const Node16 &other) {
   assert(numChildren == Node16::kMaxNodes);
   memset(index, -1, sizeof(index));
   memset(children, 0, sizeof(children));
+  memset(reverseIndex, 0, sizeof(reverseIndex));
   const auto z = InternalVersionT::zero;
   for (auto &v : childMaxVersion) {
     v = z;
@@ -568,6 +569,13 @@ inline void Node48::copyChildrenAndKeyFrom(const Node16 &other) {
         std::max(maxOfMax[i >> Node48::kMaxOfMaxShift], childMaxVersion[i]);
     ++i;
   }
+  // The reverseIndex bytes beyond numChildren are initialized so that the
+  // SIMD loads of a full 16-byte page in scan16 are well-defined (reading
+  // indeterminate bytes is UB in C++). valgrind then treats them as undefined
+  // so that it can still flag any read that relies on them.
+  VALGRIND_MAKE_MEM_UNDEFINED(reverseIndex + numChildren,
+                              sizeof(reverseIndex) -
+                                  numChildren * sizeof(reverseIndex[0]));
 }
 
 inline void Node48::copyChildrenAndKeyFrom(const Node48 &other) {
@@ -594,6 +602,7 @@ inline void Node48::copyChildrenAndKeyFrom(const Node256 &other) {
   copyCommon(*this, other);
   memset(index, -1, sizeof(index));
   memset(children, 0, sizeof(children));
+  memset(reverseIndex, 0, sizeof(reverseIndex));
   const auto z = InternalVersionT::zero;
   for (auto &v : childMaxVersion) {
     v = z;
@@ -615,6 +624,13 @@ inline void Node48::copyChildrenAndKeyFrom(const Node256 &other) {
     ++i;
   });
   memcpy(partialKey(), &other + 1, partialKeyLen);
+  // The reverseIndex bytes beyond numChildren are initialized so that the
+  // SIMD loads of a full 16-byte page in scan16 are well-defined (reading
+  // indeterminate bytes is UB in C++). valgrind then treats them as undefined
+  // so that it can still flag any read that relies on them.
+  VALGRIND_MAKE_MEM_UNDEFINED(reverseIndex + numChildren,
+                              sizeof(reverseIndex) -
+                                  numChildren * sizeof(reverseIndex[0]));
 }
 
 inline void Node256::copyChildrenAndKeyFrom(const Node48 &other) {