step C(android): compile engine.rs on Android with a stub CallEngine::start

Third incremental variable. Previously the engine module was cfg-gated
out of the Android build entirely (`#[cfg(not(target_os = "android"))]
mod engine;` in lib.rs). Now it's always compiled, so any link-time
effect of having engine.rs in the compilation unit can be measured
against the working baseline from build #19.

Changes kept deliberately small:
- lib.rs: drop the cfg gate on `mod engine;`. `use engine::CallEngine`
  stays gated because the Android-specific connect/disconnect/... stubs
  in lib.rs don't reference the type.
- engine.rs: the `wzp_client::{audio_io, call}` imports + CodecId +
  QualityProfile are gated to non-Android (they require the `audio`
  feature on wzp-client which Android doesn't pull in). On Android we
  keep only the MediaTransport import for transport.close(). The impl
  block now has two `start()` methods: the full CPAL-backed one for
  desktop, and a 6-line Android stub that returns `Err("audio engine
  not yet wired on Android")` so attempts to `connect` from the UI
  fail cleanly.

Goal: verify that linking in the compiled engine module (plus the
types it references) on Android doesn't regress the working baseline.
Home screen should still render and register_signal should still work.

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

desktop/src-tauri/src/lib.rs

@@ -6,11 +6,13 @@
     windows_subsystem = "windows"
 )]
 
-// CPAL-backed audio engine — desktop only. On Android we'll plug in an
-// oboe/AAudio backend in a later step.
-#[cfg(not(target_os = "android"))]
+// Call engine — now compiled on every platform. On desktop it runs the real
+// CPAL/VPIO audio pipeline; on Android `CallEngine::start()` currently returns
+// an error stub (see engine.rs — that's step C of the Oboe integration).
 mod engine;
 
+// CallEngine is only referenced from the non-Android connect/disconnect/etc
+// commands; the Android stubs return errors directly.
 #[cfg(not(target_os = "android"))]
 use engine::CallEngine;