fix(patches): add rstan entry to skip removed tbb_stddef.h

StanHeaders' init_threadpool_tbb.hpp unconditionally includes the legacy
<tbb/tbb_stddef.h> for TBB version detection, but that header was removed
in oneTBB 2021+ (the version StanHeaders now bundles), so rstan's Module.cpp
fails to compile when rstan is pulled in as a transitive dependency (e.g. of
ctsem).

Pre-define TBB_INTERFACE_NEW via CPPFLAGS so the header's own guard skips the
removed include and selects the modern tbb/global_control.h + tbb/task_arena.h
path that the bundled TBB provides. Preserve -I/usr/local/include since the
CPPFLAGS override replaces the R default.

local/patches/registry.json

@@ -8,5 +8,17 @@
     "makevars": {},
     "patch": "RcppParallel/disable-tbb.patch",
     "reason": "bundled Intel TBB build hangs/fails on musl (Alpine) and newer toolchains (g++ 15 on ubuntu-2604); patch unsets USE_TBB and forces -DRCPP_PARALLEL_USE_TBB=0 so RcppParallel skips the bundled build and uses the TinyThread backend"
+  },
+  {
+    "package": "rstan",
+    "versions": "*",
+    "platforms": ["*"],
+    "env": {},
+    "configure_args": [],
+    "makevars": {
+      "CPPFLAGS": "-DTBB_INTERFACE_NEW -I/usr/local/include"
+    },
+    "patch": null,
+    "reason": "StanHeaders' init_threadpool_tbb.hpp unconditionally includes the legacy <tbb/tbb_stddef.h> (removed in oneTBB 2021+) for version detection, breaking compilation of Module.cpp against the bundled oneTBB. Pre-defining TBB_INTERFACE_NEW skips that include and selects the modern tbb/global_control.h + tbb/task_arena.h path that the bundled TBB provides (-I/usr/local/include preserves the default CPPFLAGS the override replaces)"
   }
 ]