docs: document allocator override scope

docs/development.md

@@ -170,6 +170,30 @@ where glibc fortification warns. The other hardening flags stay on. Like the war
 hardening covers first-party code only; dependency binaries from the Conan cache are not rebuilt
 with these flags.
 
+### Allocators
+
+This template does not replace the system allocator, and no preset or CMake option selects
+[mimalloc](https://github.com/microsoft/mimalloc), [jemalloc](https://github.com/jemalloc/jemalloc),
+or [tcmalloc](https://github.com/google/tcmalloc). An allocator is chosen against a measured
+allocation profile, which a boilerplate does not have. Three constraints make it more than a
+link-line change here:
+
+- ASan installs its own `malloc`/`free` interceptors. An override linked on top of them either fails
+  to link or leaves the heap diagnostics silently disabled, so the `sanitize` and `sanitize-asan`
+  presets would have to gate the allocator off. UBSan does not replace the allocator, so
+  `sanitize-ubsan` can keep it.
+- The override mechanism is per platform. mimalloc's static override works on Linux; macOS needs
+  runtime interposition through `DYLD_INSERT_LIBRARIES`; Windows needs `mimalloc-redirect.dll`
+  beside the executable, which the `install(TARGETS)` rule does not ship.
+- The override is process-wide, but dependency binaries come from the Conan cache and are not
+  rebuilt. Any measurement has to cover the whole process, not the first-party targets alone.
+
+To add one, follow the path the recipe already uses for optional tools: declare a Conan option in
+[`conanfile.py`](../conanfile.py), add the requirement under `requirements()`, forward the choice to
+CMake through `tc.cache_variables` next to the `ccache` and `mold`/LLD probes in `generate()`, and
+fail configuration when `compiler.sanitizer` is `Address` or `AddressUndefinedBehavior`. Rerun
+`make lock` afterwards.
+
 ## IDE setup
 
 ### VS Code