fix(patches): build RcppParallel against system oneTBB instead of disabling TBB (#135)

## What

Replace RcppParallel's `disable-tbb` registry patch with `system-tbb`: build RcppParallel against the system oneTBB instead of stripping TBB entirely.

## Why

`disable-tbb` skipped the bundled Intel TBB build by forcing the TinyThread backend, which also removed RcppParallel's TBB linkage.
That broke every dependent that links TBB through `RcppParallelLibs()` -- `rstan` and the whole Stan cluster -- with `symbol not found: tbb::detail::r1::observe`.
This is the root cause behind the large "blocked on RcppParallel" / rstan clusters (issues #115, #120): the packages were not individually broken, they were all waiting on one TBB-linkage regression.

## Change

`system-tbb.patch` leaves `USE_TBB` unset (so the bundled build is still skipped on musl and g++ 15) but keeps the TBB backend and links the system oneTBB now shipped in the build-env images:

```
PKG_CXXFLAGS += -DRCPP_PARALLEL_USE_TBB=1 -DTBB_SUPPRESS_DEPRECATED_MESSAGES=1 -DTBB_INTERFACE_NEW
PKG_LIBS += -ltbb -ltbbmalloc
```

Scoped to `alpine` + `ubuntu-2604`, the only platforms where the bundled build fails; redhat and older ubuntus keep the bundled TBB.

## Requires

The companion image change that exports `TBB_INC`/`TBB_LIB` so `RcppParallelLibs()` hands the system-TBB flags to dependents: build-env-images PR #12. Both must ship together.

## Verified

In `build-env-alpine:3.24` with the new images:
- RcppParallel builds against oneTBB 2022, no ABI errors, patch applies cleanly to the target clone.
- `RcppParallelLibs()` returns `-L/usr/lib -Wl,-rpath,/usr/lib -ltbb -ltbbmalloc`.
- rstan links (`-ltbb -ltbbmalloc`) and loads with no missing symbol -- `* DONE (rstan)`, trial build exit 0.

Reviewed-on: #135

local/patches/RcppParallel/disable-tbb.patch

@@ -1,16 +0,0 @@
-diff --git a/src/Makevars.in b/src/Makevars.in
-index be8445f..faee771 100644
---- a/src/Makevars.in
-+++ b/src/Makevars.in
-@@ -60,7 +60,10 @@ else
- 	endif
- 
- 	ifeq ($(UNAME), Linux)
--		USE_TBB=Linux
-+		# bincraft patch: the bundled Intel TBB build hangs/fails on musl (Alpine)
-+		# and newer toolchains (g++ 15). Skip it (leave USE_TBB unset) and force the
-+		# TinyThread backend so RcppParallel still builds.
-+		PKG_CXXFLAGS += -DRCPP_PARALLEL_USE_TBB=0
- 	endif
- 
- 	ifeq ($(UNAME), SunOS)

local/patches/RcppParallel/system-tbb.patch

@@ -0,0 +1,19 @@
+diff --git a/src/Makevars.in b/src/Makevars.in
+index be8445f..7cb6c9e 100644
+--- a/src/Makevars.in
++++ b/src/Makevars.in
+@@ -60,7 +60,13 @@ else
+ 	endif
+ 
+ 	ifeq ($(UNAME), Linux)
+-		USE_TBB=Linux
++		# bincraft patch: link the system oneTBB (installed in the build-env images)
++		# instead of building the bundled Intel TBB, which fails on musl (Alpine) and
++		# newer toolchains (g++ 15). Leaving USE_TBB unset skips the bundled build;
++		# -DRCPP_PARALLEL_USE_TBB=1 keeps the TBB backend so dependents (rstan, ...)
++		# link TBB, and -ltbb/-ltbbmalloc pull the system library from default paths.
++		PKG_CXXFLAGS += -DRCPP_PARALLEL_USE_TBB=1 -DTBB_SUPPRESS_DEPRECATED_MESSAGES=1 -DTBB_INTERFACE_NEW
++		PKG_LIBS += -ltbb -ltbbmalloc
+ 	endif
+ 
+ 	ifeq ($(UNAME), SunOS)

local/patches/registry.json

@@ -2,17 +2,22 @@
   {
     "package": "RcppParallel",
     "versions": "*",
-    "platforms": ["alpine", "ubuntu-2604"],
+    "platforms": [
+      "alpine",
+      "ubuntu-2604"
+    ],
     "env": {},
     "configure_args": [],
     "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"
+    "patch": "RcppParallel/system-tbb.patch",
+    "reason": "bundled Intel TBB build hangs/fails on musl (Alpine) and newer toolchains (g++ 15 on ubuntu-2604); link the system oneTBB (now in the build-env images) instead, keeping the TBB backend so dependents (rstan, ...) link TBB"
   },
   {
     "package": "fs",
     "versions": "*",
-    "platforms": ["*"],
+    "platforms": [
+      "*"
+    ],
     "env": {},
     "configure_args": [],
     "makevars": {},
@@ -22,7 +27,9 @@
   {
     "package": "rstan",
     "versions": "*",
-    "platforms": ["*"],
+    "platforms": [
+      "*"
+    ],
     "env": {},
     "configure_args": [],
     "makevars": {