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/README.md

@@ -0,0 +1,88 @@
+# flutter_rust_bridge Hello Spike
+
+Chanora proof-of-concept. **Not product code.**
+
+| Field | Value |
+|---|---|
+| PoC name | `flutter_rust_bridge_hello` (dir kept underscored for Dart-package naming) |
+| PoC plan | [`docs/architecture/proof-of-concept-plan.md`](../../docs/architecture/proof-of-concept-plan.md) §2 |
+| Purpose | Prove the command / result / event DTO boundary between Flutter and Rust |
+| Exit criterion | "Flutter can call Rust and receive event stream data" |
+
+> **Naming note.** The PoC plan lists this spike as
+> `flutter-rust-bridge-hello`. Dart pubspec package names require
+> underscores, so the on-disk directory is `flutter_rust_bridge_hello`.
+> This is a naming convention, not a scope change.
+
+## What it does
+
+| API | Direction | Demonstrates |
+|---|---|---|
+| `greet(name)` | Dart → Rust → Dart (sync) | Typed `GreetResult` DTO + typed `GreetError` (Result-style fallible command) |
+| `counterStream(count, intervalMs)` | Rust → Dart (async stream) | Typed `CounterTick` event stream with ordered delivery and clean close |
+
+The `lib/main.dart` demo wires both into a Material 3 surface (button +
+text field + live tick list). This UI is illustrative only — the
+Chanora Design System belongs to the product Flutter app, not this PoC.
+
+## Layout
+
+```text
+flutter_rust_bridge_hello/
+  rust/                 # Rust crate (cdylib + staticlib)
+    src/
+      lib.rs            # crate root; declares api + frb_generated
+      api/
+        mod.rs
+        simple.rs       # public API (greet, counter_stream, DTOs)
+      frb_generated.rs  # autogenerated; do not edit
+    Cargo.toml
+  lib/
+    main.dart           # demo UI
+    src/rust/           # autogenerated Dart bindings; do not edit
+  test/
+    poc_verification_test.dart   # headless verification (flutter test)
+  integration_test/
+    simple_test.dart    # same checks, runs under integration_test driver
+  flutter_rust_bridge.yaml
+  pubspec.yaml
+  linux/ android/ ios/ macos/ windows/ web/
+  rust_builder/         # cargokit build hook (generated)
+```
+
+## Reproduce
+
+Requires Flutter stable (developed against 3.41.9 / Dart 3.11.5),
+`flutter_rust_bridge_codegen` 2.12.0, Rust stable (developed against 1.95).
+Linux desktop toolchain (clang, cmake, ninja, GTK3) for the build path
+proven here. Network access required for first build (cargo + pub).
+
+```bash
+# Generate bindings whenever rust/src/api/*.rs changes
+flutter_rust_bridge_codegen generate
+
+# Build for Linux desktop (also builds the cdylib via cargokit)
+flutter build linux --debug
+
+# Headless verification — exercises the same API the demo UI uses.
+LD_LIBRARY_PATH="$PWD/build/linux/x64/debug/bundle/lib:$LD_LIBRARY_PATH" \
+    flutter test test/poc_verification_test.dart
+
+# Interactive run (requires a display)
+flutter run -d linux
+```
+
+## Scope boundaries
+
+- No TeamSpeak / `tsclientlib` integration (covered by
+  `tsclientlib-connect-spike`).
+- No platform secure storage, SQLite, or audio (separate PoCs).
+- No real DTO catalogue, no error taxonomy, no event-key conventions —
+  `chanora_bridge` will own those per SAD §7.2.
+- Only the Linux desktop target has been built and verified. Android,
+  iOS, macOS, Windows are scaffolded by `flutter create` but unverified
+  here.
+
+## Verification log
+
+See `VERIFICATION.md` in this directory.