step E.1 variant: cpp_link_stdlib c++_shared → c++_static

Every E.x variant crashed identically when linked with c++_shared, even
with a 3-line cpp file that's dead-stripped from the final .so. The
crash offsets are byte-identical across E.1, E.2, E.4, and the original
full-Oboe Step E. That points at a non-code link-time delta: the
`cargo:rustc-link-lib=c++_shared` directive that adds a NEEDED entry
for libc++_shared.so to the .so's dynamic table.

Swap to c++_static — bundles libc++ directly into our .so so the
NEEDED entry disappears. If this launches cleanly, we've conclusively
proven the NEEDED libc++_shared.so is the root cause and we have a
workable linkage for any C++ we want to add to the Tauri Android build
(including the eventual Oboe audio backend).

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

desktop/src-tauri/build.rs

@@ -37,21 +37,25 @@ fn build_android_native() {
         .file("cpp/getauxval_fix.c")
         .compile("getauxval_fix");
 
-    // ─── Step E.4: minimal C++ smoke file instead of full Oboe ─────────────
+    // ─── Step E.1 with STATIC libc++ ───────────────────────────────────────
-    // The full Oboe compile (Step E, commit 4250f1b) triggered the
+    // Every cpp_smoke variant (atomic-mutex-thread / atomic-only / empty
-    // __init_tcb+4 crash at launch — even without any FFI call from Rust.
+    // function) crashed identically with c++_shared linkage — byte-
-    // Bisection jump: replace the 200+ Oboe source files with ONE tiny
+    // identical crash offsets. The cpp code was dead-stripped from the
-    // cpp/cpp_smoke.cpp that uses the same libc++ features Oboe does
+    // final .so in every case, so the only remaining delta was the
-    // (std::atomic, std::mutex, std::thread), still linked via
+    // NEEDED entry for libc++_shared.so added by
-    // libc++_shared. If this crashes too, the trigger is just "any C++
+    // cargo:rustc-link-lib=c++_shared. Theory: that NEEDED entry (and
-    // link that references libc++ threads/mutexes". If it passes, Oboe
+    // Android's dynamic linker running libc++_shared.so's init_array at
-    // itself (size, specific headers, static ctors) is the culprit.
+    // dlopen time) is the trigger.
+    //
+    // Test: swap c++_shared → c++_static. Bundles libc++ code directly
+    // into our .so, drops the NEEDED entry. If the app launches, we've
+    // proven the NEEDED libc++_shared.so is the trigger and have a
+    // working linkage for adding C++ to Tauri Android cdylibs.
     println!("cargo:rerun-if-changed=cpp/cpp_smoke.cpp");
     cc::Build::new()
         .cpp(true)
         .std("c++17")
-        // Shared libc++ — same linkage Oboe uses.
+        .cpp_link_stdlib(Some("c++_static"))
-        .cpp_link_stdlib(Some("c++_shared"))
         .file("cpp/cpp_smoke.cpp")
         .compile("wzp_cpp_smoke");