step E(android): compile full Oboe C++ bridge (not yet called from Rust)

Fifth incremental variable — and the first genuinely heavy one. Adds:
  - cpp/oboe_bridge.{h,cpp} (copied verbatim from crates/wzp-android/cpp/)
  - cpp/oboe_stub.cpp (fallback if Oboe can't be fetched)
  - build.rs now clones google/oboe@1.8.1 into OUT_DIR and compiles
    oboe_bridge.cpp + every .cpp file under oboe/src/ as a single
    static library via cc::Build, using shared libc++. Same logic as
    the legacy wzp-android build.rs.
  - libc++_shared.so gets copied from the NDK sysroot into the Tauri
    gen/android jniLibs directory so the runtime linker can find it.
  - rustc-link-lib=log / OpenSLES emitted for Oboe's Android backends.

Deliberately NOT called from Rust yet — no extern "C" FFI declarations,
no oboe_audio.rs module, the `wzp_oboe_*` symbols from the static lib
are simply present but unreferenced.

Goal: isolate whether the Oboe C++ compile + static lib link alone
(with its libc++ dependency and log/OpenSLES bindings) regresses the
working baseline. If the build still launches and renders the home
screen, we know the C++ side is clean and the actual regression is
caused by calling into Oboe at runtime (next step).

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

desktop/src-tauri/build.rs

@@ -1,3 +1,4 @@
+use std::path::PathBuf;
 use std::process::Command;
 
 fn main() {
@@ -15,24 +16,154 @@ 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");
+        build_android_native();
     }
 
     tauri_build::build()
 }
+
+fn build_android_native() {
+    // ─── Step A: cpp/hello.c sanity static lib ─────────────────────────────
+    println!("cargo:rerun-if-changed=cpp/hello.c");
+    cc::Build::new()
+        .file("cpp/hello.c")
+        .compile("wzp_hello");
+
+    // ─── Step D: getauxval_fix shim ────────────────────────────────────────
+    println!("cargo:rerun-if-changed=cpp/getauxval_fix.c");
+    cc::Build::new()
+        .file("cpp/getauxval_fix.c")
+        .compile("getauxval_fix");
+
+    // ─── Step E: full Oboe C++ bridge ──────────────────────────────────────
+    // Clones google/oboe@1.8.1 into OUT_DIR and compiles the bridge + all
+    // Oboe source files as a single static library. NOT yet called from
+    // Rust — this step only verifies that the C++ compile + link path
+    // doesn't regress the known-good build. Same approach as the legacy
+    // crates/wzp-android/build.rs, copied verbatim below.
+    println!("cargo:rerun-if-changed=cpp/oboe_bridge.cpp");
+    println!("cargo:rerun-if-changed=cpp/oboe_bridge.h");
+    println!("cargo:rerun-if-changed=cpp/oboe_stub.cpp");
+
+    match fetch_oboe() {
+        Some(oboe_path) => {
+            println!("cargo:warning=Building with Oboe from {:?}", oboe_path);
+
+            let mut build = cc::Build::new();
+            build
+                .cpp(true)
+                .std("c++17")
+                // Shared libc++ — static pulls broken libc stubs that crash
+                // in .so libraries (getauxval, __init_tcb, pthread_create).
+                // Google's official NDK guidance.
+                .cpp_link_stdlib(Some("c++_shared"))
+                .include("cpp")
+                .include(oboe_path.join("include"))
+                .include(oboe_path.join("src"))
+                .define("WZP_HAS_OBOE", None)
+                .file("cpp/oboe_bridge.cpp");
+
+            add_cpp_files_recursive(&mut build, &oboe_path.join("src"));
+            build.compile("oboe_bridge");
+        }
+        None => {
+            println!("cargo:warning=Oboe not found, building with stub");
+            cc::Build::new()
+                .cpp(true)
+                .std("c++17")
+                .cpp_link_stdlib(Some("c++_shared"))
+                .file("cpp/oboe_stub.cpp")
+                .include("cpp")
+                .compile("oboe_bridge");
+        }
+    }
+
+    // Copy libc++_shared.so next to libwzp_desktop_lib.so in the Tauri
+    // jniLibs directory so the dynamic linker can resolve it at runtime.
+    if let Ok(ndk) = std::env::var("ANDROID_NDK_HOME")
+        .or_else(|_| std::env::var("NDK_HOME"))
+    {
+        let (triple, abi) = match target_os_abi() {
+            Some(v) => v,
+            None => ("aarch64-linux-android", "arm64-v8a"),
+        };
+        let lib_dir = format!(
+            "{ndk}/toolchains/llvm/prebuilt/linux-x86_64/sysroot/usr/lib/{triple}"
+        );
+        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/{abi}");
+            if std::fs::create_dir_all(&jni_dir).is_ok() {
+                let _ = std::fs::copy(&shared_so, format!("{jni_dir}/libc++_shared.so"));
+                println!("cargo:warning=Copied libc++_shared.so to {jni_dir}");
+            }
+        }
+    }
+
+    // Oboe requires Android log + OpenSLES backends
+    println!("cargo:rustc-link-lib=log");
+    println!("cargo:rustc-link-lib=OpenSLES");
+}
+
+fn target_os_abi() -> Option<(&'static str, &'static str)> {
+    let target = std::env::var("TARGET").ok()?;
+    if target.contains("aarch64") {
+        Some(("aarch64-linux-android", "arm64-v8a"))
+    } else if target.contains("armv7") {
+        Some(("arm-linux-androideabi", "armeabi-v7a"))
+    } else if target.contains("x86_64") {
+        Some(("x86_64-linux-android", "x86_64"))
+    } else if target.contains("i686") {
+        Some(("i686-linux-android", "x86"))
+    } else {
+        None
+    }
+}
+
+/// Recursively add all .cpp files from a directory to a cc::Build.
+fn add_cpp_files_recursive(build: &mut cc::Build, dir: &std::path::Path) {
+    if !dir.is_dir() {
+        return;
+    }
+    for entry in std::fs::read_dir(dir).unwrap() {
+        let entry = entry.unwrap();
+        let path = entry.path();
+        if path.is_dir() {
+            add_cpp_files_recursive(build, &path);
+        } else if path.extension().map_or(false, |e| e == "cpp") {
+            build.file(&path);
+        }
+    }
+}
+
+/// Try to find or fetch Oboe headers + source (v1.8.1).
+fn fetch_oboe() -> Option<PathBuf> {
+    let out_dir = PathBuf::from(std::env::var("OUT_DIR").unwrap());
+    let oboe_dir = out_dir.join("oboe");
+
+    if oboe_dir.join("include").join("oboe").join("Oboe.h").exists() {
+        return Some(oboe_dir);
+    }
+
+    let status = Command::new("git")
+        .args([
+            "clone",
+            "--depth=1",
+            "--branch=1.8.1",
+            "https://github.com/google/oboe.git",
+            oboe_dir.to_str().unwrap(),
+        ])
+        .status();
+
+    match status {
+        Ok(s) if s.success() && oboe_dir.join("include").join("oboe").join("Oboe.h").exists() => {
+            Some(oboe_dir)
+        }
+        _ => None,
+    }
+}