## Motivation
`rstan` fails to compile when it is pulled in as a transitive dependency (e.g. of `ctsem`), aborting the dependency install:
```
/mnt/cache/R-pkgs/StanHeaders/include/stan/math/prim/core/init_threadpool_tbb.hpp:9:10:
fatal error: tbb/tbb_stddef.h: No such file or directory
9 | #include <tbb/tbb_stddef.h>
```
StanHeaders' `init_threadpool_tbb.hpp` unconditionally includes the legacy `<tbb/tbb_stddef.h>` to detect the TBB version, but that header was removed in oneTBB 2021+, which is the TBB that StanHeaders now bundles.
## Fix
Add an `rstan` entry to `local/patches/registry.json` that pre-defines `TBB_INTERFACE_NEW` via `CPPFLAGS`.
The header's own guard skips the removed include when that macro is already defined and selects the modern `tbb/global_control.h` + `tbb/task_arena.h` path that the bundled oneTBB actually provides:
```cpp
#ifndef TBB_INTERFACE_NEW
#include <tbb/tbb_stddef.h> // removed in oneTBB 2021+
#if TBB_VERSION_MAJOR >= 2020
#define TBB_INTERFACE_NEW
#endif
#endif
#ifdef TBB_INTERFACE_NEW
#include <tbb/global_control.h> // taken when we pre-define the macro
#include <tbb/task_arena.h>
```
This is a makevars-tier override (no source diff), applied through bincraft's `R_MAKEVARS_USER`, so `CPPFLAGS` stays additive and does not clobber rstan's own `PKG_CPPFLAGS`.
`-I/usr/local/include` is kept because the override replaces the default `CPPFLAGS`.
## Files changed
- `local/patches/registry.json`: add the `rstan` entry (`versions: "*"`, `platforms: ["*"]`, `makevars.CPPFLAGS = "-DTBB_INTERFACE_NEW -I/usr/local/include"`).
## Behaviour change
When bincraft resolves `rstan` (direct or transitive) it builds a patched binary with `TBB_INTERFACE_NEW` defined, so `rstan` (and dependents such as `ctsem`) compile against the bundled oneTBB.
The scope is `["*"]` because the failure is bound to StanHeaders' bundled TBB version, not the OS or toolchain.
Reviewed-on: #114
pat-s
04059f20e3