android: dpi_scale, scissor, JNI safe-insets, touch input

Four Android UX wins landing together; all verified end-to-end on a
Pixel 7 Pro (board fills width, info-panel text renders, status bar
inset honored, tap-to-select + tap-to-move plays 1. e4).

- AndroidPlatform.init reads density via AConfiguration_getDensity
  (app->config at offset 32) and sets dpi_scale = density / 160. The
  hardcoded 1.0 had been making every logical unit equal one physical
  pixel; ChessBoardView's 520-default size_that_fits fallback then
  rendered at ~half the framebuffer width on the device, and glyphs
  rasterized at literal 11-13 physical pixels were essentially invisible
  on a 2340-tall display.
- gles3.sx set_scissor un-stubbed; with dpi_scale right the renderer
  feeds in valid pixel bounds and the Y-flip math lands inside the
  framebuffer.
- New library/vendors/sx_android_jni/sx_android_jni.c walks
  activity -> window -> decorView -> rootWindowInsets via JNI and
  publishes the system-bar insets. safe_insets() lazy-queries the
  first call after EGL is up (decor view isn't attached at bootstrap).
- sx_android_install_input_handler sets app->onInputEvent; sx-side
  sx_android_input_event translates AMotionEvent DOWN/MOVE/UP/CANCEL
  into existing mouse_down/mouse_moved/mouse_up Events so the chess
  board's tap-to-select + ScrollView drag path Just Works. Coordinates
  divided by dpi_scale so layout-side hit tests match. poll_events
  drains its slice after returning (mirrors the SDL pattern).
- src/imports.zig now routes #import c { #source / #include } paths
  through the same chain as #import (importing dir -> CWD -> stdlib
  search paths). Lets library-owned C helpers like the JNI bridge
  live in sx/library/vendors/ without forcing consumers to vendor a
  copy. Existing CWD-relative consumer layouts (chess's vendors/...)
  still resolve first, so no regression.

86/86 regression tests pass.

library/vendors/sx_android_jni/sx_android_jni.c

@@ -0,0 +1,94 @@
+// JNI helpers used by modules/platform/android.sx. Kept in the library
+// so consumers don't need to vendor an identically-named copy. The sx
+// compiler resolves `#source "vendors/..."` against the stdlib search
+// paths in addition to the consumer's project root.
+
+#ifdef __ANDROID__
+#include <android/native_activity.h>
+#include <android/input.h>
+#include <jni.h>
+
+// Mirror of struct android_app (NDK 29 / arm64) up to the fields we touch.
+// Avoids depending on the glue header in the sx library compile path.
+struct sx_android_app_min {
+    void* userData;
+    void (*onAppCmd)(struct sx_android_app_min* app, int cmd);
+    int (*onInputEvent)(struct sx_android_app_min* app, AInputEvent* event);
+    // ...rest of struct ignored; we only assign onInputEvent.
+};
+
+// Install an sx-side handler as `app->onInputEvent`. native_app_glue's
+// process_input loop calls this for every AInputEvent it pulls off the
+// input queue. Returning 1 marks the event as consumed.
+void sx_android_install_input_handler(void* app,
+    int (*handler)(void* app, void* event)) {
+    if (app == 0) return;
+    struct sx_android_app_min* a = (struct sx_android_app_min*)app;
+    a->onInputEvent = (int (*)(struct sx_android_app_min*, AInputEvent*))handler;
+}
+
+// Query system-bar insets (status bar, nav bar) via JNI:
+//   activity → getWindow() → getDecorView()
+//            → getRootWindowInsets()
+//            → getSystemWindowInset[Top|Left|Bottom|Right]()
+// Out params receive physical-pixel insets; the sx caller divides by
+// dpi_scale to convert to logical units. Falls back to zeros if the
+// view isn't attached yet (e.g. called before the first frame).
+void sx_android_query_safe_insets(void* activity_ptr,
+                                  int* out_top, int* out_left,
+                                  int* out_bottom, int* out_right) {
+    *out_top = 0; *out_left = 0; *out_bottom = 0; *out_right = 0;
+    if (activity_ptr == 0) return;
+
+    ANativeActivity* act = (ANativeActivity*)activity_ptr;
+    JavaVM* vm = act->vm;
+    if (vm == 0) return;
+
+    JNIEnv* env = 0;
+    int already_attached = 1;
+    if ((*vm)->GetEnv(vm, (void**)&env, JNI_VERSION_1_6) != JNI_OK) {
+        already_attached = 0;
+        if ((*vm)->AttachCurrentThread(vm, &env, 0) != 0) return;
+    }
+
+    jobject activity_obj = act->clazz;
+    jclass activity_cls = (*env)->GetObjectClass(env, activity_obj);
+
+    jmethodID m_getWindow = (*env)->GetMethodID(env, activity_cls,
+        "getWindow", "()Landroid/view/Window;");
+    if (m_getWindow == 0) goto done;
+    jobject window = (*env)->CallObjectMethod(env, activity_obj, m_getWindow);
+    if (window == 0) goto done;
+
+    jclass window_cls = (*env)->GetObjectClass(env, window);
+    jmethodID m_getDecorView = (*env)->GetMethodID(env, window_cls,
+        "getDecorView", "()Landroid/view/View;");
+    if (m_getDecorView == 0) goto done;
+    jobject decor = (*env)->CallObjectMethod(env, window, m_getDecorView);
+    if (decor == 0) goto done;
+
+    jclass view_cls = (*env)->GetObjectClass(env, decor);
+    jmethodID m_getRootInsets = (*env)->GetMethodID(env, view_cls,
+        "getRootWindowInsets", "()Landroid/view/WindowInsets;");
+    if (m_getRootInsets == 0) goto done;
+    jobject insets = (*env)->CallObjectMethod(env, decor, m_getRootInsets);
+    if (insets == 0) goto done;
+
+    jclass insets_cls = (*env)->GetObjectClass(env, insets);
+    jmethodID m_top = (*env)->GetMethodID(env, insets_cls,
+        "getSystemWindowInsetTop", "()I");
+    jmethodID m_left = (*env)->GetMethodID(env, insets_cls,
+        "getSystemWindowInsetLeft", "()I");
+    jmethodID m_bottom = (*env)->GetMethodID(env, insets_cls,
+        "getSystemWindowInsetBottom", "()I");
+    jmethodID m_right = (*env)->GetMethodID(env, insets_cls,
+        "getSystemWindowInsetRight", "()I");
+    if (m_top)    *out_top    = (*env)->CallIntMethod(env, insets, m_top);
+    if (m_left)   *out_left   = (*env)->CallIntMethod(env, insets, m_left);
+    if (m_bottom) *out_bottom = (*env)->CallIntMethod(env, insets, m_bottom);
+    if (m_right)  *out_right  = (*env)->CallIntMethod(env, insets, m_right);
+
+done:
+    if (!already_attached) (*vm)->DetachCurrentThread(vm);
+}
+#endif