docs: document building SITL with sanitizers (ASan/TSan)

Add a test_and_ci page covering the PX4_ASAN / PX4_TSAN / PX4_MSAN / PX4_UBSAN
build variables, the Linux `sudo sysctl vm.mmap_rnd_bits=28` workaround for the
sanitizer runtime's startup memory mapping, and the TSan suppressions file
(test/sitl_tsan.supp) for the Gazebo simulation transport. Linked from SUMMARY.md
and the test_and_ci index.

docs/en/test_and_ci/index.md

@@ -7,6 +7,7 @@ Test topics include:
 
 - [Test Flights](../test_and_ci/test_flights.md) - How to make test flights (e.g. to [test PRs](../contribute/code.md#pull-requests))
 - [Unit Tests](../test_and_ci/unit_tests.md)
+- [Sanitizers](../test_and_ci/sanitizers.md) - Build SITL with ASan/TSan to catch memory errors and data races
 - [Continuous Integration (CI)](../test_and_ci/continous_integration.md)
 - [Integration Testing](../test_and_ci/integration_testing.md)
   - [MAVSDK Integration Testing](../test_and_ci/integration_testing_mavsdk.md)

docs/en/test_and_ci/sanitizers.md

@@ -0,0 +1,73 @@
+# Sanitizers
+
+[Sanitizers](https://github.com/google/sanitizers) are compiler-instrumentation tools that detect whole classes of bugs at runtime. PX4 can build [SITL](../simulation/index.md) with them to catch problems that are otherwise hard to reproduce:
+
+- **AddressSanitizer (ASan)** — out-of-bounds accesses, use-after-free, memory leaks.
+- **ThreadSanitizer (TSan)** — data races and lock-order inversions.
+- **MemorySanitizer (MSan)** — reads of uninitialized memory (Clang only).
+- **UndefinedBehaviorSanitizer (UBSan)** — undefined behaviour (signed overflow, misaligned access, …).
+
+:::info
+Sanitizers are only available for POSIX/SITL builds, where the host compiler instruments the code. They are not used on the NuttX flight-controller targets.
+:::
+
+## Building and running
+
+Set the matching environment variable when building SITL. It selects the corresponding `CMAKE_BUILD_TYPE`:
+
+| Variable     | Sanitizer                     |
+| ------------ | ----------------------------- |
+| `PX4_ASAN`   | AddressSanitizer              |
+| `PX4_TSAN`   | ThreadSanitizer               |
+| `PX4_MSAN`   | MemorySanitizer               |
+| `PX4_UBSAN`  | UndefinedBehaviorSanitizer    |
+
+For example, to build and run SITL under ThreadSanitizer:
+
+```sh
+PX4_TSAN=1 make px4_sitl_default
+```
+
+To build and run the unit and integration test suite under a sanitizer:
+
+```sh
+PX4_TSAN=1 make tests
+```
+
+:::warning
+A sanitizer is a distinct `CMAKE_BUILD_TYPE`, so switching to or from one rebuilds from scratch. Sanitized binaries are larger and run several times slower (ASan/TSan roughly 2–5×), and lockstep timing changes, so expect tests to take longer.
+:::
+
+## Linux: ASLR fix
+
+On recent Linux kernels the default address-space-layout randomization entropy is too high for the sanitizer runtimes' shadow-memory mapping, and the process aborts at startup with e.g.:
+
+```
+FATAL: ThreadSanitizer: unexpected memory mapping 0x... - 0x...
+```
+
+Lower the mmap randomization (needed for both ASan and TSan), once per boot:
+
+```sh
+sudo sysctl vm.mmap_rnd_bits=28
+```
+
+To make it persistent across reboots:
+
+```sh
+echo 'vm.mmap_rnd_bits=28' | sudo tee /etc/sysctl.d/99-sanitizers.conf
+```
+
+If `28` still fails on your kernel, try `26`.
+
+## ThreadSanitizer suppressions
+
+Running SITL with the Gazebo bridge under TSan reports many data races inside the third-party simulation transport (ZeroMQ and gz-transport) and the PX4↔Gazebo bridge glue. None of this is in the flight code path or the SIH-based CI, and it can't be fixed here, so it is filtered with a suppressions file at `test/sitl_tsan.supp`:
+
+```sh
+TSAN_OPTIONS="suppressions=$PWD/test/sitl_tsan.supp" PX4_TSAN=1 make px4_sitl_default gz_x500
+```
+
+:::warning
+Only add third-party / simulation symbols to the suppressions file — never flight-code symbols. A race detector is only useful at zero reports; suppressing your own code blinds it to real bugs.
+:::