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/events.rs

@@ -86,6 +86,19 @@ pub struct AchievementUnlockedEvent(pub AchievementRecord);
 #[derive(Message, Debug, Clone, Copy, Default)]
 pub struct ManualSyncRequestEvent;
 
+/// Request to toggle the pause overlay. Fired by the HUD "Pause" button so
+/// the same toggle path runs whether the player presses `Esc` or clicks.
+/// Consumed by `pause_plugin::toggle_pause`, which honours the same drag /
+/// game-over / selection guards either way.
+#[derive(Message, Debug, Clone, Copy, Default)]
+pub struct PauseRequestEvent;
+
+/// Request to toggle the help / controls overlay. Fired by the HUD "Help"
+/// button alongside the existing `F1` accelerator so the overlay is
+/// reachable without a keyboard. Consumed by `help_plugin::toggle_help_screen`.
+#[derive(Message, Debug, Clone, Copy, Default)]
+pub struct HelpRequestEvent;
+
 /// Fired by `SyncPlugin` after a pull task resolves and the merged result has
 /// been persisted to disk. `Ok(SyncResponse)` carries the merged payload plus
 /// any `ConflictReport`s the merge produced. `Err(String)` carries a