feat(engine): add Undo, Pause, Help UI buttons in HUD action bar

Continues the UI-first pass started by the New Game button. Per the
design principle in CLAUDE.md / ARCHITECTURE.md §1, every player action
must be reachable from a visible UI control with the keyboard shortcut
as an optional accelerator. Refactor the single New Game button into a
flex-row "action bar" anchored top-right with four buttons: Undo,
Pause, Help, New Game (left → right; New Game rightmost as the most
consequential action).

Plumbing:
- New `PauseRequestEvent` and `HelpRequestEvent` in events.rs.
- pause_plugin::toggle_pause reads either Esc or PauseRequestEvent so
  the button and the keyboard accelerator drive the same code path
  (with the existing drag / game-over / selection guards).
- help_plugin::toggle_help_screen reads either F1 or HelpRequestEvent;
  also fix the stale module-doc claim that H toggles help (it's F1 —
  H is bound to hint cycle in input_plugin).
- hud_plugin now spawns four ActionButton-marked buttons via a
  ChildSpawnerCommands helper, with one click handler per button
  firing its respective request event. A single
  paint_action_buttons system covers hover/pressed colour for all of
  them via the shared ActionButton marker. The click handlers
  defensively re-register their request events so the plugin works in
  isolation under MinimalPlugins (tests). add_message is idempotent.
- ARCHITECTURE.md HudPlugin row updated to call out the action bar.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

solitaire_engine/src/lib.rs

@@ -67,14 +67,17 @@ pub use font_plugin::{FontPlugin, FontResource};
 pub use cursor_plugin::CursorPlugin;
 pub use events::{
     AchievementUnlockedEvent, CardFaceRevealedEvent, CardFlippedEvent, DrawRequestEvent,
-    ForfeitEvent, GameWonEvent, HintVisualEvent, InfoToastEvent, ManualSyncRequestEvent,
-    MoveRejectedEvent, MoveRequestEvent, NewGameConfirmEvent, NewGameRequestEvent,
-    StateChangedEvent, SyncCompleteEvent, UndoRequestEvent, XpAwardedEvent,
+    ForfeitEvent, GameWonEvent, HelpRequestEvent, HintVisualEvent, InfoToastEvent,
+    ManualSyncRequestEvent, MoveRejectedEvent, MoveRequestEvent, NewGameConfirmEvent,
+    NewGameRequestEvent, PauseRequestEvent, StateChangedEvent, SyncCompleteEvent,
+    UndoRequestEvent, XpAwardedEvent,
 };
 pub use game_plugin::{ConfirmNewGameScreen, GameMutation, GameOverScreen, GamePlugin, GameStatePath};
 pub use help_plugin::{HelpPlugin, HelpScreen};
 pub use home_plugin::{HomePlugin, HomeScreen};
-pub use hud_plugin::{HudAutoComplete, HudPlugin};
+pub use hud_plugin::{
+    ActionButton, HelpButton, HudAutoComplete, HudPlugin, NewGameButton, PauseButton, UndoButton,
+};
 pub use leaderboard_plugin::{LeaderboardPlugin, LeaderboardResource, LeaderboardScreen};
 pub use input_plugin::InputPlugin;
 pub use onboarding_plugin::{OnboardingPlugin, OnboardingScreen};