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/pubspec.yaml

@@ -0,0 +1,34 @@
+name: rust_lib_flutter_rust_bridge_hello
+description: "Utility to build Rust code"
+version: 0.0.1
+publish_to: none
+
+environment:
+  sdk: '>=3.3.0 <4.0.0'
+  flutter: '>=3.3.0'
+
+dependencies:
+  flutter:
+    sdk: flutter
+  plugin_platform_interface: ^2.0.2
+
+dev_dependencies:
+  ffi: ^2.0.2
+  ffigen: ^11.0.0
+  flutter_test:
+    sdk: flutter
+  flutter_lints: ^2.0.0
+
+flutter:
+  plugin:
+    platforms:
+      android:
+        ffiPlugin: true
+      ios:
+        ffiPlugin: true
+      linux:
+        ffiPlugin: true
+      macos:
+        ffiPlugin: true
+      windows:
+        ffiPlugin: true