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.

CMakeLists.txt

@@ -5,7 +5,7 @@ project(
   DESCRIPTION
     "A data structure for optimistic concurrency control on ranges of bitwise-lexicographically-ordered keys."
   HOMEPAGE_URL "https://git.weaselab.dev/weaselab/conflict-set"
-  LANGUAGES C CXX ASM)
+  LANGUAGES C CXX)
 set(CMAKE_CXX_STANDARD 20)
 
 file(WRITE ${CMAKE_CURRENT_BINARY_DIR}/version.txt ${PROJECT_VERSION})
@@ -130,19 +130,7 @@ endif()
 
 set(CMAKE_CXX_IMPLICIT_LINK_LIBRARIES "")
 
-# Architecture-specific SIMD assembly. These functions operate on
-# potentially-indeterminate memory, which is UB in C++ but well-defined in
-# assembly.
-set(SIMD_ASM_FILES)
-if(CMAKE_SYSTEM_PROCESSOR STREQUAL x86_64 AND NOT USE_SIMD_FALLBACK)
-  set(SIMD_ASM_FILES ${CMAKE_CURRENT_SOURCE_DIR}/simd_x86_64.S)
-elseif((CMAKE_SYSTEM_PROCESSOR STREQUAL aarch64 OR CMAKE_SYSTEM_PROCESSOR
-                                                   STREQUAL arm64)
-       AND NOT USE_SIMD_FALLBACK)
-  set(SIMD_ASM_FILES ${CMAKE_CURRENT_SOURCE_DIR}/simd_aarch64.S)
-endif()
-
-add_library(${PROJECT_NAME}-object OBJECT ConflictSet.cpp ${SIMD_ASM_FILES})
+add_library(${PROJECT_NAME}-object OBJECT ConflictSet.cpp)
 target_compile_options(${PROJECT_NAME}-object PRIVATE -fno-exceptions
                                                       -fvisibility=hidden)
 target_include_directories(${PROJECT_NAME}-object
@@ -245,7 +233,7 @@ if(CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR AND BUILD_TESTING)
   endif()
 
   # ad hoc testing
-  add_executable(conflict_set_main ConflictSet.cpp ${SIMD_ASM_FILES})
+  add_executable(conflict_set_main ConflictSet.cpp)
   target_include_directories(conflict_set_main
                              PRIVATE ${CMAKE_CURRENT_SOURCE_DIR}/include)
   target_compile_definitions(conflict_set_main PRIVATE ENABLE_MAIN)
@@ -261,7 +249,7 @@ if(CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR AND BUILD_TESTING)
     cmake_pop_check_state()
 
     if(HAS_LIB_FUZZER)
-      add_executable(conflict_set_fuzz_test ConflictSet.cpp ${SIMD_ASM_FILES})
+      add_executable(conflict_set_fuzz_test ConflictSet.cpp)
       target_include_directories(conflict_set_fuzz_test
                                  PRIVATE ${CMAKE_CURRENT_SOURCE_DIR}/include)
       target_compile_definitions(conflict_set_fuzz_test PRIVATE ENABLE_FUZZ)
@@ -273,8 +261,7 @@ if(CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR AND BUILD_TESTING)
   endif()
 
   # whitebox tests asan+ubsan
-  add_executable(fuzz_driver ConflictSet.cpp FuzzTestDriver.cpp
-                             ${SIMD_ASM_FILES})
+  add_executable(fuzz_driver ConflictSet.cpp FuzzTestDriver.cpp)
   target_compile_options(fuzz_driver PRIVATE ${TEST_FLAGS})
   if(NOT CMAKE_CROSSCOMPILING)
     target_compile_options(fuzz_driver PRIVATE -fsanitize=address,undefined)
@@ -290,8 +277,7 @@ if(CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR AND BUILD_TESTING)
 
   # whitebox tests msan
   if(MSAN_TOOLCHAIN_PATH)
-    add_executable(fuzz_driver_msan ConflictSet.cpp FuzzTestDriver.cpp
-                                    ${SIMD_ASM_FILES})
+    add_executable(fuzz_driver_msan ConflictSet.cpp FuzzTestDriver.cpp)
     target_compile_options(fuzz_driver_msan PRIVATE ${TEST_FLAGS})
     if(NOT CMAKE_CROSSCOMPILING)
       target_compile_options(
@@ -319,8 +305,7 @@ if(CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR AND BUILD_TESTING)
 
   # tsan tests
   if(NOT CMAKE_CROSSCOMPILING AND NOT DISABLE_TSAN)
-    add_executable(tsan_driver ConflictSet.cpp FuzzTestDriver.cpp
-                               ${SIMD_ASM_FILES})
+    add_executable(tsan_driver ConflictSet.cpp FuzzTestDriver.cpp)
     target_compile_options(tsan_driver PRIVATE ${TEST_FLAGS} -fsanitize=thread)
     target_link_options(tsan_driver PRIVATE -fsanitize=thread)
     target_compile_definitions(tsan_driver PRIVATE ENABLE_FUZZ THREAD_TEST)