feat(engine): reserve top band for HUD so it stops crowding the cards

Player report: the action button bar (Menu / Undo / Pause / Help /
Modes / New Game) and Score / Moves / Timer text were sharing the
same vertical band as the stock + foundation row, with no visual
separation. The HUD read as part of the play surface.

Two-part fix:

1. layout.rs reserves HUD_BAND_HEIGHT (64 px) at the top of the
   window. Card-grid math takes that off the available vertical
   budget so cards still fit; top_y shifts down by the same amount.
   New layout test pins the reservation. Existing
   worst_case_tableau_fits_vertically tests verify the height-budget
   arithmetic still holds.

2. hud_plugin.rs spawns a translucent purple band (BG_HUD_BAND, new
   token in ui_theme.rs at the BG_BASE hue with 0.70 alpha) filling
   that reserved zone. Z-index sits one rung below Z_HUD so action
   buttons paint on top while the band reads as their container. The
   band's bottom edge lines up with the top edge of the highest
   playable card, so the buttons feel anchored to a "tools strip"
   rather than floating in the play area.

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

solitaire_engine/src/ui_theme.rs

@@ -48,6 +48,13 @@ pub const BG_ELEVATED_PRESSED: Color = Color::srgb(0.149, 0.082, 0.357);
 /// them. `rgba(13, 7, 28, 0.85)`.
 pub const SCRIM: Color = Color::srgba(0.051, 0.027, 0.110, 0.85);
 
+/// Translucent fill for the top-of-window HUD band painted by
+/// `hud_plugin::spawn_hud_band`. Same midnight-purple hue as `BG_BASE`,
+/// but at 0.70 alpha so the green felt reads through subtly — enough
+/// to mark the band as "UI" without feeling like a hard chrome strip.
+/// `rgba(26, 15, 46, 0.70)`.
+pub const BG_HUD_BAND: Color = Color::srgba(0.102, 0.059, 0.180, 0.70);
+
 /// Primary text — warm off-white with a hint of purple to fit the
 /// midnight palette without feeling clinical. `#F5F0FF`.
 pub const TEXT_PRIMARY: Color = Color::srgb(0.961, 0.941, 1.000);