docs(build): describe the ThreadSanitizer suppressions and guest memory reports

docs/dev/BUILD.md

@@ -135,6 +135,10 @@ SDL and FreeType settings forced by the root `CMakeLists.txt` cannot be overridd
 
 With `ANYPS5_USE_SYSTEM_DEPS=ON` those libraries come from the system (on Debian and Ubuntu: `libsdl2-dev`, `libfreetype-dev`, `libvulkan-dev`, `spirv-headers`, `glslang-dev`, `glslang-tools` and `spirv-tools`). A missing package, Vulkan-Headers older than 1.4.310 (the agc driver uses `VK_KHR_maintenance8`), or an SDL2 without the HIDAPI joystick driver (sdl2-compat over SDL3, or a build with HIDAPI disabled, which breaks the pad and disk-audio tests), fails the configure with an error and never falls back to the submodule. The forced SDL and FreeType settings from the root `CMakeLists.txt` do not apply with the flag ON: the system SDL2 comes with whatever its distribution enabled. The prx libraries are shared objects, so SDL2 is linked from the system as a shared library; the default build keeps its own static SDL2. `ffmpeg-core`, `libjpeg-turbo`, `stb`, `VulkanMemoryAllocator`, `LibAtrac9`, `ogg` and `vorbis` always stay bundled and still need `git submodule update --init`.
 
+## ThreadSanitizer
+
+A ThreadSanitizer build runs its tests with `TSAN_OPTIONS=suppressions=$PWD/tools/tsan-suppressions.txt`, which silences reports from Mesa's lavapipe and PulseAudio: those system libraries are not instrumented, so ThreadSanitizer cannot see their locking. Guest memory written by the emulated GPU while a guest thread polls it, such as labels and fences, is shared without C++ atomics on purpose, as on the console, so ThreadSanitizer also reports those accesses.
+
 ## Quick syntax checks
 
 After configuring a build directory, use `tools/check_compile.py` to check the given source files. The tool reads their compile commands from `compile_commands.json` or Ninja, preserving compilation options while dropping object and dependency output options. It handles `-MF`, `-MT`, `-MQ` and `-MJ` with separate or attached values so that unused output options do not cause false failures under `-Werror`.