feat(engine): contextual one-time tips — stall hint, radial menu teach (Phase I)

Two situation-fired teaches, each shown exactly once and recorded in
Settings like shown_achievement_onboarding:

- Stall tip: 45s with no board change in an active, started game
  (clock frozen while paused / a modal is open) points at Hint with
  platform-correct copy (H on desktop, bottom-bar Hint on touch).
- Radial teach: a player 15 moves into a game who has never opened the
  radial menu learns the long-press / right-click gesture. Organic
  radial use marks the tip done silently — nobody is taught what they
  already know.

Tips ride the queued InfoToastEvent path, so they render in the unified
toast stack and never interrupt play. This completes Phase I.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

solitaire_engine/src/lib.rs

@@ -15,6 +15,7 @@ pub mod avatar_plugin;
 pub mod card_animation;
 pub mod card_plugin;
 pub mod challenge_plugin;
+pub mod contextual_tips_plugin;
 pub mod core_game_plugin;
 pub mod cursor_plugin;
 pub mod daily_challenge_plugin;
@@ -94,6 +95,7 @@ pub use card_plugin::{
 pub use challenge_plugin::{
     CHALLENGE_UNLOCK_LEVEL, ChallengeAdvancedEvent, ChallengePlugin, challenge_progress_label,
 };
+pub use contextual_tips_plugin::ContextualTipsPlugin;
 pub use core_game_plugin::CoreGamePlugin;
 pub use cursor_plugin::CursorPlugin;
 pub use daily_challenge_plugin::{