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>

desktop/src-tauri/build.rs

@@ -1,7 +1,9 @@
 use std::process::Command;
 
 fn main() {
-    // ─── Embedded git hash ─────────────────────────────────────────────────
+    // Capture short git hash so the running app can prove which build it is.
+    // Falls back to "unknown" if git isn't available (e.g. when building from
+    // a tarball without a .git dir).
     let git_hash = Command::new("git")
         .args(["rev-parse", "--short", "HEAD"])
         .output()
@@ -15,75 +17,10 @@ fn main() {
     println!("cargo:rerun-if-changed=../../.git/HEAD");
     println!("cargo:rerun-if-changed=../../.git/refs/heads");
 
-    // ─── Step A: single trivial cpp/hello.c compiled via cc::Build ─────────
-    // ─── Step D: also compile getauxval_fix.c (legacy wzp-android shim) ────
-    // getauxval_fix.c overrides the broken static getauxval stub that
-    // compiler-rt pulls in for Android targets. It's been shipping in the
-    // legacy wzp-android .so for months without issue, so including it here
-    // is low-risk — but it's an incremental variable we want to isolate.
-    let target = std::env::var("TARGET").unwrap_or_default();
-    if target.contains("android") {
-        println!("cargo:rerun-if-changed=cpp/hello.c");
-        cc::Build::new()
-            .file("cpp/hello.c")
-            .compile("wzp_hello");
-
-        println!("cargo:rerun-if-changed=cpp/getauxval_fix.c");
-        cc::Build::new()
-            .file("cpp/getauxval_fix.c")
-            .compile("getauxval_fix");
-
-        // Step D+1: identical-content clone of hello.c as a third cc::Build
-        // static library. Kept around as a sanity check: if this C compile
-        // suddenly started crashing, we'd know the environment regressed.
-        println!("cargo:rerun-if-changed=cpp/hello2.c");
-        cc::Build::new()
-            .file("cpp/hello2.c")
-            .compile("wzp_hello2");
-
-        // ─── minSdkVersion theory test: the original E.1 crashing cpp ──────
-        // Re-add the smallest crashing variant (cpp_smoke.cpp with cpp(true)
-        // + cpp_link_stdlib("c++_shared")) on top of the working Step D+1
-        // baseline. The only additional variable compared to the previous
-        // crashing runs is tauri.conf.json bundle.android.minSdkVersion=26,
-        // which may make tauri-cli stop hardcoding API 24 in its rustc
-        // invocation. If THIS build launches, the minSdkVersion fix is
-        // validated and we can proceed with Oboe integration.
-        println!("cargo:rerun-if-changed=cpp/cpp_smoke.cpp");
-        cc::Build::new()
-            .cpp(true)
-            .std("c++17")
-            .cpp_link_stdlib(Some("c++_shared"))
-            .file("cpp/cpp_smoke.cpp")
-            .compile("wzp_cpp_smoke");
-
-        // Per rust-lang/rust#104707 + the android-ndk advice: force the
-        // linker to keep bionic symbols (pthread_create, __init_tcb) as
-        // UND dynamic references resolved against libc.so at runtime,
-        // not bound locally from libc.a that cc-rs + cpp(true) drag in.
-        // llvm-nm confirmed these symbols were landing in our .so as
-        // LOCAL (lowercase t), which is exactly the bug.
-        println!("cargo:rustc-link-arg=-Wl,--exclude-libs,ALL");
-        println!("cargo:rustc-link-arg=-Wl,--no-whole-archive");
-
-        // Copy libc++_shared.so from the NDK sysroot to gen/android jniLibs
-        // so the runtime linker can find it at dlopen time (it's now in the
-        // .so's NEEDED list thanks to cpp_link_stdlib("c++_shared") above).
-        if let Ok(ndk) = std::env::var("ANDROID_NDK_HOME").or_else(|_| std::env::var("NDK_HOME")) {
-            let lib_dir = format!(
-                "{ndk}/toolchains/llvm/prebuilt/linux-x86_64/sysroot/usr/lib/aarch64-linux-android"
-            );
-            println!("cargo:rustc-link-search=native={lib_dir}");
-            let shared_so = format!("{lib_dir}/libc++_shared.so");
-            if std::path::Path::new(&shared_so).exists() {
-                let manifest = std::env::var("CARGO_MANIFEST_DIR").unwrap_or_default();
-                let jni_dir = format!("{manifest}/gen/android/app/src/main/jniLibs/arm64-v8a");
-                if std::fs::create_dir_all(&jni_dir).is_ok() {
-                    let _ = std::fs::copy(&shared_so, format!("{jni_dir}/libc++_shared.so"));
-                }
-            }
-        }
-    }
+    // No cc::Build of ANY kind on Android — all C++ lives in the standalone
+    // `wzp-native` crate which is built separately with cargo-ndk and loaded
+    // via libloading at runtime. See docs/incident-tauri-android-init-tcb.md
+    // for why this split exists.
 
     tauri_build::build()
 }