fix(engine): correct Android help hint label from → to ! (M-17)

The HUD buttons section in the Android controls reference showed "→"
(right-arrow) for the Hint action, but the actual on-screen button is
labelled "!" (ASCII exclamation). Extract ANDROID_HINT_LABEL from
hud_plugin so both the spawn path and the help text share a single
source of truth. Add a cfg(android) regression test that asserts the
hint row's key string matches the const.

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

solitaire_engine/src/hud_plugin.rs

@@ -298,6 +298,11 @@ pub struct HelpButton;
 #[derive(Component, Debug)]
 pub struct HintButton;
 
+/// Android HUD label for the Hint button — shared with the help screen's
+/// controls reference so both always agree.
+#[cfg(target_os = "android")]
+pub(crate) const ANDROID_HINT_LABEL: &str = "!";
+
 /// Marker on the "Modes" action button. Click toggles the [`ModesPopover`]
 /// (a small dropdown panel) below the action bar. Each popover row starts
 /// the corresponding game mode.
@@ -856,7 +861,7 @@ fn spawn_action_buttons(
         /* undo */  "\u{2190}",  // ←  leftwards arrow (Arrows block, confirmed FiraMono)
         /* pause */ "||",        // || ASCII double-pipe — ‖ (U+2016) absent from FiraMono
         /* help */  "?",
-        /* hint */  "!",         // !  attention/alert — semantically: "look here"
+        /* hint */  ANDROID_HINT_LABEL,
         /* modes */ "M",          // plain ASCII — U+21BB and U+21C4 both render as tofu on FiraMono
         /* new */   "+",
     );