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/.metadata

@@ -0,0 +1,45 @@
+# This file tracks properties of this Flutter project.
+# Used by Flutter tool to assess capabilities and perform upgrades etc.
+#
+# This file should be version controlled and should not be manually edited.
+
+version:
+  revision: "00b0c91f06209d9e4a41f71b7a512d6eb3b9c694"
+  channel: "stable"
+
+project_type: app
+
+# Tracks metadata for the flutter migrate command
+migration:
+  platforms:
+    - platform: root
+      create_revision: 00b0c91f06209d9e4a41f71b7a512d6eb3b9c694
+      base_revision: 00b0c91f06209d9e4a41f71b7a512d6eb3b9c694
+    - platform: android
+      create_revision: 00b0c91f06209d9e4a41f71b7a512d6eb3b9c694
+      base_revision: 00b0c91f06209d9e4a41f71b7a512d6eb3b9c694
+    - platform: ios
+      create_revision: 00b0c91f06209d9e4a41f71b7a512d6eb3b9c694
+      base_revision: 00b0c91f06209d9e4a41f71b7a512d6eb3b9c694
+    - platform: linux
+      create_revision: 00b0c91f06209d9e4a41f71b7a512d6eb3b9c694
+      base_revision: 00b0c91f06209d9e4a41f71b7a512d6eb3b9c694
+    - platform: macos
+      create_revision: 00b0c91f06209d9e4a41f71b7a512d6eb3b9c694
+      base_revision: 00b0c91f06209d9e4a41f71b7a512d6eb3b9c694
+    - platform: web
+      create_revision: 00b0c91f06209d9e4a41f71b7a512d6eb3b9c694
+      base_revision: 00b0c91f06209d9e4a41f71b7a512d6eb3b9c694
+    - platform: windows
+      create_revision: 00b0c91f06209d9e4a41f71b7a512d6eb3b9c694
+      base_revision: 00b0c91f06209d9e4a41f71b7a512d6eb3b9c694
+
+  # User provided section
+
+  # List of Local paths (relative to this file) that should be
+  # ignored by the migrate tool.
+  #
+  # Files that are not part of the templates will be ignored by default.
+  unmanaged_files:
+    - 'lib/main.dart'
+    - 'ios/Runner.xcodeproj/project.pbxproj'