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/android/settings.gradle.kts

@@ -0,0 +1,26 @@
+pluginManagement {
+    val flutterSdkPath =
+        run {
+            val properties = java.util.Properties()
+            file("local.properties").inputStream().use { properties.load(it) }
+            val flutterSdkPath = properties.getProperty("flutter.sdk")
+            require(flutterSdkPath != null) { "flutter.sdk not set in local.properties" }
+            flutterSdkPath
+        }
+
+    includeBuild("$flutterSdkPath/packages/flutter_tools/gradle")
+
+    repositories {
+        google()
+        mavenCentral()
+        gradlePluginPortal()
+    }
+}
+
+plugins {
+    id("dev.flutter.flutter-plugin-loader") version "1.0.0"
+    id("com.android.application") version "8.11.1" apply false
+    id("org.jetbrains.kotlin.android") version "2.2.20" apply false
+}
+
+include(":app")