feat(engine): in-game HUD — score, move count, elapsed time, mode badge

Adds HudPlugin with a persistent top-left overlay that shows score,
move count, and elapsed time during every game. A mode badge highlights
DAILY, CHALLENGE, ZEN, or TIME ATTACK when the game is not in Classic
mode. HUD updates whenever GameStateResource changes (moves and per-second
time ticks) without a separate polling system.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

solitaire_engine/src/lib.rs

@@ -10,6 +10,7 @@ pub mod daily_challenge_plugin;
 pub mod events;
 pub mod game_plugin;
 pub mod help_plugin;
+pub mod hud_plugin;
 pub mod leaderboard_plugin;
 pub mod input_plugin;
 pub mod layout;
@@ -43,6 +44,7 @@ pub use events::{
 };
 pub use game_plugin::{GameMutation, GamePlugin, GameStatePath};
 pub use help_plugin::{HelpPlugin, HelpScreen};
+pub use hud_plugin::HudPlugin;
 pub use leaderboard_plugin::{LeaderboardPlugin, LeaderboardResource, LeaderboardScreen};
 pub use input_plugin::InputPlugin;
 pub use onboarding_plugin::{OnboardingPlugin, OnboardingScreen};