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/Cargo.toml

@@ -17,6 +17,10 @@ path = "src/main.rs"
 
 [build-dependencies]
 tauri-build = { version = "2", features = [] }
+# Step A: minimal cc addition — lets build.rs compile cpp/hello.c on Android.
+# build-dependencies are resolved against the HOST, not the target, so this
+# line is unconditional (the actual cc::Build call is gated on TARGET).
+cc = "1"
 
 [dependencies]
 tauri = { version = "2", features = [] }