feat(poc/bridge): add flutter_rust_bridge hello spike

Proof-of-concept proving the Flutter/Rust bridge exit criterion from
docs/architecture/proof-of-concept-plan.md §2:
  "Flutter can call Rust and receive event stream data."

The spike exposes one synchronous fallible command (greet) returning
a typed GreetResult / GreetError DTO, and one async event stream
(counter_stream) emitting typed CounterTick events. The Flutter app
demonstrates both flows on a Material 3 surface; the headless test
suite in test/poc_verification_test.dart exercises the same API
directly through dart:ffi.

Verified on 2026-05-13 (Linux desktop, Flutter 3.41.9 / Dart 3.11.5,
flutter_rust_bridge 2.12.0, Rust 1.95). All three tests pass:
  - greet() returns typed result for valid input
  - greet() surfaces typed error for empty input
  - counterStream() delivers the expected event sequence

Authority: PoC plan §2, DEC-014 (typed Flutter/Rust bridge),
SAD-068, SDD-079, SysDes-049.
Naming note: the PoC plan lists this as flutter-rust-bridge-hello,
but Dart pubspec.yaml package names require underscores; the
directory uses underscores accordingly.
Not product code; not promoted into chanora_bridge.

Layout note: includes the full Flutter platform scaffold (android,
ios, macos, windows, web, linux). Only the Linux desktop target has
been built and verified.

poc/flutter_rust_bridge_hello/rust_builder/android/.gitignore

@@ -0,0 +1,9 @@
+*.iml
+.gradle
+/local.properties
+/.idea/workspace.xml
+/.idea/libraries
+.DS_Store
+/build
+/captures
+.cxx

poc/flutter_rust_bridge_hello/rust_builder/android/build.gradle

@@ -0,0 +1,56 @@
+// The Android Gradle Plugin builds the native code with the Android NDK.
+
+group 'com.flutter_rust_bridge.rust_lib_flutter_rust_bridge_hello'
+version '1.0'
+
+buildscript {
+    repositories {
+        google()
+        mavenCentral()
+    }
+
+    dependencies {
+        // The Android Gradle Plugin knows how to build native code with the NDK.
+        classpath 'com.android.tools.build:gradle:7.3.0'
+    }
+}
+
+rootProject.allprojects {
+    repositories {
+        google()
+        mavenCentral()
+    }
+}
+
+apply plugin: 'com.android.library'
+
+android {
+    if (project.android.hasProperty("namespace")) {
+        namespace 'com.flutter_rust_bridge.rust_lib_flutter_rust_bridge_hello'
+    }
+
+    // Bumping the plugin compileSdkVersion requires all clients of this plugin
+    // to bump the version in their app.
+    compileSdkVersion 33
+
+    // Use the NDK version
+    // declared in /android/app/build.gradle file of the Flutter project.
+    // Replace it with a version number if this plugin requires a specfic NDK version.
+    // (e.g. ndkVersion "23.1.7779620")
+    ndkVersion android.ndkVersion
+
+    compileOptions {
+        sourceCompatibility JavaVersion.VERSION_1_8
+        targetCompatibility JavaVersion.VERSION_1_8
+    }
+
+    defaultConfig {
+        minSdkVersion 19
+    }
+}
+
+apply from: "../cargokit/gradle/plugin.gradle"
+cargokit {
+    manifestDir = "../../rust"
+    libname = "rust_lib_flutter_rust_bridge_hello"
+}

poc/flutter_rust_bridge_hello/rust_builder/android/settings.gradle

@@ -0,0 +1 @@
+rootProject.name = 'rust_lib_flutter_rust_bridge_hello'

poc/flutter_rust_bridge_hello/rust_builder/android/src/main/AndroidManifest.xml

@@ -0,0 +1,3 @@
+<manifest xmlns:android="http://schemas.android.com/apk/res/android"
+  package="com.flutter_rust_bridge.rust_lib_flutter_rust_bridge_hello">
+</manifest>