refactor(android): split C++ into wzp-native cdylib, loaded at runtime

Phase 1 of the big refactor. Escape the Tauri Android
__init_tcb+4 symbol leak (rust-lang/rust#104707) by making
wzp-desktop's Android .so pure Rust — ZERO cc::Build, no cpp/ files,
no C++ in the rustc link step. All future C++ (Oboe audio bridge)
lives in a new standalone cdylib crate `wzp-native` which is built
with cargo-ndk (the same path the legacy wzp-android crate uses
successfully on the same phone + same NDK), copied into Tauri's
gen/android/app/src/main/jniLibs at build time, and dlopened by
wzp-desktop at runtime via libloading.

Changes in this commit:
- NEW crate crates/wzp-native/ with crate-type = ["cdylib"] only
  (no staticlib, no rlib — rust#104707 shows mixing staticlib with
  cdylib leaks non-exported symbols, which is the original bug
  source). Phase 1 scaffold has TWO extern "C" functions:
    wzp_native_version() -> i32            (returns 42)
    wzp_native_hello(buf, cap) -> usize    (writes a string)
  So we can verify dlopen + dlsym + cross-.so FFI end-to-end
  before adding any real C++.
- desktop/src-tauri/cpp/ directory DELETED (7 files gone).
- desktop/src-tauri/build.rs reduced to just the git hash capture
  + tauri_build::build(). No more cc::Build of any kind.
- desktop/src-tauri/Cargo.toml: drop cc from build-dependencies,
  add libloading = "0.8" as an Android-only runtime dep.
- desktop/src-tauri/src/lib.rs Builder::setup() now (on Android only)
  dlopens libwzp_native.so, calls wzp_native_version() and
  wzp_native_hello(), and logs the result:
    "wzp-native dlopen OK: version=42 msg=\"hello from wzp-native\""
  If this log appears in logcat when the app launches and the home
  screen still renders, the split-cdylib pipeline is validated and
  Phase 2 (port the Oboe bridge into wzp-native) can proceed.
- scripts/build-tauri-android.sh: insert a `cargo ndk -t arm64-v8a
  build --release -p wzp-native` step before `cargo tauri android
  build`, with `-o desktop/src-tauri/gen/android/app/src/main/jniLibs`
  so the resulting libwzp_native.so lands in the place gradle will
  package into the final APK.
- Workspace Cargo.toml: add crates/wzp-native to [workspace] members.

Phase 2 (separate commit, only if Phase 1 works):
- Copy cpp/oboe_bridge.{h,cpp} + getauxval_fix.c from the legacy
  wzp-android crate into crates/wzp-native/cpp/.
- Add cc = "1" as a build-dependency on wzp-native (safe: it's a
  single-cdylib crate with no staticlib, so no symbol leak).
- Add build.rs that compiles the Oboe C++ and the wzp-native Rust
  FFI exposes the audio start/stop/read/write functions.
- wzp-desktop::engine.rs dlopens wzp-native at CallEngine::start,
  uses its audio functions instead of CPAL on Android.

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

Cargo.toml

@@ -10,6 +10,7 @@ members = [
     "crates/wzp-client",
     "crates/wzp-web",
     "crates/wzp-android",
+    "crates/wzp-native",
     "desktop/src-tauri",
 ]