step A(android): add cc=1 build-dep + compile single trivial hello.c

First incremental variable on the path back to Oboe integration. Changes
are deliberately minimal: add cc = "1" to [build-dependencies] (cargo
build-deps resolve against the host so the line is unconditional), and
on the Android target run a single cc::Build step that compiles
cpp/hello.c — a 6-line file that defines one function (`wzp_hello_stub`)
that is never called from Rust.

Goal: verify that merely introducing a C static library into the .so
via cc::Build does not regress the working build (#17, commit 5309938
= build #6 behaviour: launches, renders home screen, registers on
relay). If this build still works, we know cc::Build pipelines alone
are fine and can move to the next variable.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

desktop/src-tauri/cpp/hello.c

@@ -0,0 +1,14 @@
+/* hello.c — minimal C file compiled via cc::Build on Android.
+ *
+ * Step A of the incremental Oboe integration: this file exists only to
+ * exercise the cc::Build → static lib → rustc-link pipeline and prove
+ * that introducing any C static library into our .so doesn't by itself
+ * trigger the tao::ndk_glue pthread_create crash we hit on earlier
+ * attempts. The function is deliberately never called from Rust.
+ */
+
+#include <stdint.h>
+
+int32_t wzp_hello_stub(void) {
+    return 42;
+}