feat(smoke): add canonical SDK smoke against sdk_demo_backend

[Taure]

Summary

Adds the canonical SDK smoke test from sdk_demo_backend/SMOKE.md and a CI workflow that brings up the demo backend and runs it on every PR + push to main.

The smoke imports only from package:flame_asobi/flame_asobi.dart, so a green run also proves the bridge package's re-export contract still works (AsobiClient, AsobiRealtime, the realtime event streams, PlayerState).

The Flame mixins (HasAsobi, HasAsobiInput, AsobiNetworkSync, ...) require a running FlameGame loop and are out of scope for a headless smoke. Proving the underlying client contract holds is enough to gate releases on the upstream SDK contract.

What the smoke verifies

Per SMOKE.md:

1. Auth + WS connect — register two players (smoke_a_<ts>_<rand>, smoke_b_...), open WebSockets.
2. Matchmaker -> match.matched — both queue with mode "demo", both receive a match.matched event with the same match_id within 10 s.
3. match.input -> match.state — capture x_initial from the first match.state, send {move_x: 1, move_y: 0} via realtime.sendMatchInput({...}), assert a subsequent match.state shows x > x_initial + 10 within 3 s.

asobi dependency

Pinned to a git ref on main:

asobi:
  git:
    url: https://github.com/widgrensit/asobi-dart
    ref: main

The smoke depends on the post-PR-#14 sendMatchInput(Map<String, dynamic>) signature (typed MatchInput was removed). asobi-dart 0.1.0 on pub.dev still has the old typed API. The git pin should be reverted to a pub.dev version (>= 0.2.0) once asobi-dart cuts a release.

Out of scope

• The pre-existing 70+ analyzer errors in test/ (class -> mixin migration). Untouched.
• Publishing prep — keeping the version at 0.1.0 and the git dep is intentional for now; a follow-up PR should swap to a pub.dev version before the next release.

CI

.github/workflows/smoke.yml checks out flame_asobi + widgrensit/sdk_demo_backend, brings up docker compose, sets up Flutter (needed because flame_asobi transitively imports package:flutter), runs dart run smoke_tests/smoke.dart against http://localhost:8084, and dumps backend logs on failure.

Test plan

• CI smoke job goes green on this PR.
• Manual: cd ../sdk_demo_backend && docker compose up -d && cd - && ASOBI_URL=http://localhost:8084 dart run smoke_tests/smoke.dart -> [smoke] PASS.
• Once asobi-dart publishes >= 0.2.0, replace the git pin with a pub.dev constraint.