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/integration_test/simple_test.dart

@@ -0,0 +1,48 @@
+// Chanora PoC verification — flutter_rust_bridge_hello.
+//
+// Verifies the PoC exit criterion from
+// `docs/architecture/proof-of-concept-plan.md` §2:
+//     "Flutter can call Rust and receive event stream data."
+//
+// This is a Flutter integration test (runs on a real Flutter engine on the
+// Linux desktop target). It exercises the FRB-generated Dart API directly
+// rather than driving the demo UI — that is sufficient evidence for the
+// PoC exit criterion and is robust against future UI changes.
+
+import 'package:flutter_rust_bridge_hello/src/rust/api/simple.dart';
+import 'package:flutter_rust_bridge_hello/src/rust/frb_generated.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:integration_test/integration_test.dart';
+
+void main() {
+  IntegrationTestWidgetsFlutterBinding.ensureInitialized();
+  setUpAll(() async => await RustLib.init());
+
+  group('Chanora FRB hello PoC', () {
+    test('greet() returns typed result for valid input', () {
+      final r = greet(name: 'Chanora');
+      expect(r.message, contains('Hello, Chanora!'));
+      // Sanity-check the typed numeric field round-trips.
+      expect(r.elapsedMicros, isA<BigInt>());
+      expect(r.elapsedMicros.toInt(), greaterThanOrEqualTo(0));
+    });
+
+    test('greet() surfaces typed error for empty input', () {
+      // FRB maps the Rust `Err(GreetError { .. })` to a Dart exception.
+      expect(() => greet(name: '   '), throwsA(isA<GreetError>()));
+    });
+
+    test('counterStream() delivers the expected event sequence', () async {
+      const count = 4;
+      final events = <CounterTick>[];
+      await for (final tick in counterStream(count: count, intervalMs: 50)) {
+        events.add(tick);
+      }
+      expect(events, hasLength(count));
+      for (var i = 0; i < count; i++) {
+        expect(events[i].seq.toInt(), i);
+        expect(events[i].note, 'tick $i/$count');
+      }
+    });
+  });
+}