fix: RcppParallel disable-TBB source patch (env var was a no-op) (#106)

## Summary

Fixes the RcppParallel patch, which was a **no-op** and left the build hanging.

The previous registry entry set `env: { RCPP_PARALLEL_USE_TBB: "0" }`. But `RCPP_PARALLEL_USE_TBB` is a **compile-time `-D` flag** in RcppParallel's Makevars — it is never read from the environment. So the override did nothing: `USE_TBB=Linux` (hardcoded from `uname`) still triggered the **bundled Intel TBB build**, which hangs/fails on musl (Alpine) and newer toolchains (g++ 15 on ubuntu-2604). The `Applying patch …` log only meant the env was set, not that it had any effect.

## Fix

Replace the env entry with a **source patch** (`local/patches/RcppParallel/disable-tbb.patch`) on `src/Makevars.in` that, on Linux:

- leaves `USE_TBB` unset → the whole bundled-TBB build/link path is skipped (no hang), and
- forces `PKG_CXXFLAGS += -DRCPP_PARALLEL_USE_TBB=0` → the sources compile the **TinyThread** backend (needed because `RcppParallel.h` otherwise auto-defaults TBB on for glibc Linux).

## Verification

In a Linux container, applying the patch and running `R CMD INSTALL RcppParallel`:

```
bundled_TBB_build=0      # bundled TBB build never runs
* DONE (RcppParallel)    # installs via TinyThread
```

## Note

bincraft's `apply_source_patch` shells out to `patch`. If a build-env image lacks the `patch` tool (common on Alpine), the patch will report "did not apply cleanly" and fall back to an unpatched (hanging) build. If that happens, the follow-up is to switch bincraft's patch application to `git apply` (git is always present) — happy to do that if needed.

Reviewed-on: #106

local/patches/RcppParallel/disable-tbb.patch

@@ -0,0 +1,16 @@
+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)