build(android): link libdocument_detector against shared libc++

[farfromrefug]

Summary

• libdocument_detector.so was built with -DANDROID_STL=c++_static, so each ABI carried a private copy of libc++/libc++abi. The napi runtime (@nativescript/android-quickjs-ng) already ships libc++_shared.so for all 4 ABIs, so that copy was pure duplication.
• Switch to -DANDROID_STL=c++_shared. The existing pickFirst 'lib/*/libc++_shared.so' rules in App_Resources/*/Android/app.gradle keep exactly one copy in the APK — verified, APK still has a single libc++_shared.so (the runtime's).
• Align src-native/.../build.gradle ndkVersion with the one the real build actually uses (27.3.13750724, from before-plugins.gradle).

Stripped libdocument_detector.so, before -> after:

ABI	before	after	saved
arm64-v8a	7132720	6695728	427 KB
armeabi-v7a	4503864	4247608	250 KB
x86	8374820	7955876	409 KB
x86_64	9591536	9174512	407 KB

Bulk of it is locale.cpp.o (145 KB) and cxa_demangle.cpp.o (113 KB).

ABI compatibility

The runtime's libc++_shared.so is NDK r28 (clang 21); the plugin builds with NDK 27.3 (clang 18). Both use the std::__ndk1 inline namespace, as do the prebuilt libopencv_*.a / libtesseract.a / libleptonica.a. Only 12 symbols differ between the two shared libs (all std::pmr / GC-declare) and none are referenced by libdocument_detector.so. After the switch the .so has zero unresolved non-libc symbols; all 170 libc++ imports are exported by the runtime's copy. libNativeScript.so's own 113 libc++ imports resolve against either copy, so pickFirst is safe whichever wins.

Testing

Built and installed the arm64 debug APK on an emulator:

• libdocument_detector.so now lists NEEDED libc++_shared.so on all 4 ABIs
• nativeloader: Load ... libdocument_detector.so ... : ok — no dlopen/symbol errors
• Camera preview with live document detection runs; capture produces a detected corner quad

Still needs a check on a real arm64 device: the emulator run crashes in kleidicv::sve2::remap_s16point5 (SIGILL) during crop, which is the pre-existing Apple-Silicon-emulator SVE issue documented in cpp/CMakeLists.txt (DISABLE_SVE), not related to this change — with -PdisableSVE the crop path gets past it. OCR (tesseract), QR scan (cardwallet flavour, -PwithQRCode) and PDF export have not been exercised end-to-end yet.