android: keyboard height tracking via JNI/FFI bridge

- C bridge (keyboard_bridge.c) stores keyboard state in globals.
  Kotlin writes via JNI, Dart reads via FFI — zero async delay,
  same architecture as iOS.
- WindowInsetsAnimation.Callback tracks open/close per-frame.
- OnGlobalLayoutListener catches silent height changes (emoji
  keyboard resize, floating keyboard toggle).
- Dart animation replay stays iOS-only; Android reads native
  per-frame values directly.
- Cleaned up old Java stub, updated build.gradle for Kotlin + CMake
  with 16KB page alignment (Android 15+).
- Example app rewritten to demonstrate UxKeyboard usage.

android/src/main/jni/CMakeLists.txt

@@ -0,0 +1,8 @@
+cmake_minimum_required(VERSION 3.10)
+project(ux_keyboard)
+
+add_library(ux_keyboard SHARED keyboard_bridge.c)
+target_link_libraries(ux_keyboard log)
+
+# 16KB page size support (required for Android 15+)
+target_link_options(ux_keyboard PRIVATE "-Wl,-z,max-page-size=16384")

android/src/main/jni/keyboard_bridge.c

@@ -0,0 +1,53 @@
+#include <jni.h>
+#include <stdint.h>
+
+// Shared state — Kotlin writes, Dart reads via FFI
+static double g_keyboard_height = 0;
+static double g_system_height = 0;
+static int32_t g_is_tracking = 0;
+static double g_anim_target = 0;
+static double g_anim_duration = 0;
+static int32_t g_anim_generation = 0;
+
+// --- Dart FFI reads (same signatures as iOS) ---
+
+double ux_keyboard_height(void) { return g_keyboard_height; }
+double ux_system_keyboard_height(void) { return g_system_height; }
+int32_t ux_is_tracking(void) { return g_is_tracking; }
+double ux_keyboard_anim_target(void) { return g_anim_target; }
+double ux_keyboard_anim_duration(void) { return g_anim_duration; }
+int32_t ux_keyboard_anim_gen(void) { return g_anim_generation; }
+
+// --- Stubs for Dart FFI parity with iOS ---
+
+void ux_enable_interactive_dismiss(double inset) { (void)inset; }
+void ux_disable_interactive_dismiss(void) {}
+
+// --- Kotlin JNI writes ---
+
+JNIEXPORT void JNICALL
+Java_io_swipelab_ux_KeyboardBridge_nSetHeight(JNIEnv *env, jclass cls, jdouble h) {
+    g_keyboard_height = h;
+}
+
+JNIEXPORT void JNICALL
+Java_io_swipelab_ux_KeyboardBridge_nSetSystemHeight(JNIEnv *env, jclass cls, jdouble h) {
+    g_system_height = h;
+}
+
+JNIEXPORT jdouble JNICALL
+Java_io_swipelab_ux_KeyboardBridge_nGetSystemHeight(JNIEnv *env, jclass cls) {
+    return g_system_height;
+}
+
+JNIEXPORT void JNICALL
+Java_io_swipelab_ux_KeyboardBridge_nSetTracking(JNIEnv *env, jclass cls, jint v) {
+    g_is_tracking = v;
+}
+
+JNIEXPORT void JNICALL
+Java_io_swipelab_ux_KeyboardBridge_nSetAnim(JNIEnv *env, jclass cls, jdouble target, jdouble duration) {
+    g_anim_target = target;
+    g_anim_duration = duration;
+    g_anim_generation++;
+}