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/VERIFICATION.md

@@ -0,0 +1,68 @@
+# Verification record — `flutter_rust_bridge_hello`
+
+## Result
+
+PASS. The PoC exit criterion (from
+`docs/architecture/proof-of-concept-plan.md` §2: "Flutter can call Rust
+and receive event stream data") is met.
+
+## Environment
+
+| Field | Value |
+|---|---|
+| Date | 2026-05-13 |
+| Host OS | Linux (Arch, kernel 7.0.5-arch1-1, x86_64) |
+| Flutter | 3.41.9 stable (Dart 3.11.5) |
+| Rust toolchain | stable 1.95.0 |
+| `flutter_rust_bridge` (Rust + Dart) | 2.12.0 |
+| `flutter_rust_bridge_codegen` | 2.12.0 |
+| Target | linux desktop (x86_64-unknown-linux-gnu) |
+
+## Reproduction
+
+```bash
+# 1. Generate Dart bindings + Rust glue from rust/src/api.
+flutter_rust_bridge_codegen generate
+
+# 2. Build the Linux desktop bundle (this builds the cdylib too, via cargokit).
+flutter build linux --debug
+
+# 3. Run the headless verification test. The cdylib lives in the build bundle,
+#    so we point dart:ffi at it via LD_LIBRARY_PATH.
+LD_LIBRARY_PATH="$PWD/build/linux/x64/debug/bundle/lib:$LD_LIBRARY_PATH" \
+    flutter test test/poc_verification_test.dart
+```
+
+## Observed result
+
+```
+00:00 +0: loading /home/.../test/poc_verification_test.dart
+00:00 +0: (setUpAll)
+00:00 +0: Chanora FRB hello PoC greet() returns typed result for valid input
+00:00 +1: Chanora FRB hello PoC greet() surfaces typed error for empty input
+00:00 +2: Chanora FRB hello PoC counterStream() delivers the expected event sequence
+00:00 +3: (tearDownAll)
+00:00 +3: All tests passed!
+```
+
+## What the verification proves
+
+| Exit-criterion sub-claim | Evidence |
+|---|---|
+| Dart can call Rust | `greet(name: 'Chanora')` returns a `GreetResult` whose `message` field originated in Rust. |
+| Typed result DTO crosses the boundary | `GreetResult.elapsedMicros` is a typed `BigInt` (Rust `u64`) deserialised correctly on the Dart side. |
+| Typed error DTO crosses the boundary | `greet(name: '   ')` throws a `GreetError` instance on the Dart side (`isA<GreetError>()`); the Rust `Err(...)` arm is reached and reified as a Dart exception. |
+| Dart can receive a Rust event stream | `counterStream(count: 4, intervalMs: 50)` yields exactly 4 ordered `CounterTick` events with `seq = 0..3` and matching `note` payloads. |
+| Streams close cleanly | The `await for` loop terminates on its own without timeout. |
+
+## Scope honestly NOT validated by this spike
+
+- Performance characteristics of the bridge under load.
+- Android, iOS, macOS, Windows targets.
+- Lifecycle behaviour across Flutter hot-restart or Dart isolate boundaries.
+- Cancellation semantics (Dart listener disconnect while Rust still produces).
+- Behaviour with large/complex DTO graphs.
+- Backpressure on the event stream.
+- The full Chanora bridge DTO catalogue (owned by `chanora_bridge`).
+- The "schema-controlled" half of DEC-014 — this PoC uses macro-driven
+  generation only.