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/ios/Classes/dummy_file.c

@@ -0,0 +1 @@
+// This is an empty file to force CocoaPods to create a framework.

poc/flutter_rust_bridge_hello/rust_builder/ios/rust_lib_flutter_rust_bridge_hello.podspec

@@ -0,0 +1,45 @@
+#
+# To learn more about a Podspec see http://guides.cocoapods.org/syntax/podspec.html.
+# Run `pod lib lint rust_lib_flutter_rust_bridge_hello.podspec` to validate before publishing.
+#
+Pod::Spec.new do |s|
+  s.name             = 'rust_lib_flutter_rust_bridge_hello'
+  s.version          = '0.0.1'
+  s.summary          = 'A new Flutter FFI plugin project.'
+  s.description      = <<-DESC
+A new Flutter FFI plugin project.
+                       DESC
+  s.homepage         = 'http://example.com'
+  s.license          = { :file => '../LICENSE' }
+  s.author           = { 'Your Company' => 'email@example.com' }
+
+  # This will ensure the source files in Classes/ are included in the native
+  # builds of apps using this FFI plugin. Podspec does not support relative
+  # paths, so Classes contains a forwarder C file that relatively imports
+  # `../src/*` so that the C sources can be shared among all target platforms.
+  s.source           = { :path => '.' }
+  s.source_files = 'Classes/**/*'
+  s.dependency 'Flutter'
+  s.platform = :ios, '11.0'
+
+  # Flutter.framework does not contain a i386 slice.
+  s.pod_target_xcconfig = { 'DEFINES_MODULE' => 'YES', 'EXCLUDED_ARCHS[sdk=iphonesimulator*]' => 'i386' }
+  s.swift_version = '5.0'
+
+  s.script_phase = {
+    :name => 'Build Rust library',
+    # First argument is relative path to the `rust` folder, second is name of rust library
+    :script => 'sh "$PODS_TARGET_SRCROOT/../cargokit/build_pod.sh" ../../rust rust_lib_flutter_rust_bridge_hello',
+    :execution_position => :before_compile,
+    :input_files => ['${BUILT_PRODUCTS_DIR}/cargokit_phony'],
+    # Let XCode know that the static library referenced in -force_load below is
+    # created by this build step.
+    :output_files => ["${BUILT_PRODUCTS_DIR}/librust_lib_flutter_rust_bridge_hello.a"],
+  }
+  s.pod_target_xcconfig = {
+    'DEFINES_MODULE' => 'YES',
+    # Flutter.framework does not contain a i386 slice.
+    'EXCLUDED_ARCHS[sdk=iphonesimulator*]' => 'i386',
+    'OTHER_LDFLAGS' => '-force_load ${BUILT_PRODUCTS_DIR}/librust_lib_flutter_rust_bridge_hello.a',
+  }
+end