feat(engine): replace docked touch bar with floating glass tab bar

Touch layout only (desktop bar unchanged): a floating glass pill
(Undo / Draw / Hint / Pause) plus a detached circular Menu button,
margin above the bottom edge on top of the safe-area inset. Draw is
the persistently expanded accent pill (icon + label); other buttons
are icon-only and slide their label out while pressed, snapping under
reduce-motion. Buttons keep the existing marker components so click
handlers, tooltips, focus ring, and chrome toggle work unchanged.
Icons: None degrades to text-fallback labels (MinimalPlugins tests /
SVG regression), keeping the bar fully usable.

Toast stack clearance now derives from TAB_BAR_CLEARANCE_PX; the
legacy touch bar spawn path and primary-button metrics are removed.

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

solitaire_engine/src/hud_plugin/mod.rs

@@ -12,11 +12,13 @@ use bevy::window::WindowResized;
 mod fx;
 mod interaction;
 mod spawn;
+pub(crate) mod tab_bar;
 mod updates;
 
 pub use fx::*;
 use interaction::*;
 use spawn::*;
+use tab_bar::*;
 use updates::*;
 
 // On wasm32 AvatarPlugin is gated out; define a placeholder type so the
@@ -595,6 +597,12 @@ impl Plugin for HudPlugin {
                     handle_menu_backdrop_click,
                     close_menu_popover_on_escape,
                     paint_action_buttons,
+                    paint_glass_tab_buttons,
+                    drive_tab_expansion,
+                    // Writes tab-label TextColor and reads TextFont — entity-
+                    // disjoint from the UiTextFx readout writers (tab labels
+                    // vs HUD readouts), which Bevy can't prove, so declare it.
+                    animate_tab_expansion.ambiguous_with(UiTextFx),
                 )
                     .chain()
                     .in_set(HudButtons)