regression check: revert build.rs to exact Step D state

Verify the Step D baseline still launches after the environment mutations
we may have caused during the E bisection (docker image rebuild, tauri-cli
version drift, etc). Build.rs is now byte-identical to commit a852cad
(Step D) except for the git hash capture block that already existed at
that point.

If this launches cleanly → the cpp_smoke addition genuinely breaks
something, bisection continues.
If this crashes → the environment regressed between Step D and now,
and we need to rebuild the docker image to an earlier snapshot.

desktop/src-tauri/cpp/cpp_smoke.c

@@ -1,9 +0,0 @@
-// cpp_smoke.c — Step E.minus-1: same content as the crashing .cpp file
-// but as plain C. No extern "C" linkage spec (that's C++-only syntax;
-// in C every function has C linkage by default). If this crashes we
-// know cc::Build is being wrongly accused — the trigger must be more
-// general than C++ mode.
-
-int wzp_cpp_hello(void) {
-    return 42;
-}