refactor(graphics): introduce Z-value constants and layer manager

Add centralized Z-value management for Qt's stacking order system. This
replaces scattered magic numbers with semantic constants, making the rendering
hierarchy explicit and maintainable.

The architecture defines three layers:
- Zone layer (0-999): Zone backgrounds and containers
- Card layer (1-40M): Cards on the table zone using position-based formula
- Overlay layer (200M+): UI elements that always render above cards

ZValueLayerManager provides validation utilities and the overlayZValue()
function for computing overlay Z-values. ZValues namespace provides the
semantic constants used throughout the codebase.

The 5x gap between card max (40M) and overlay base (200M) provides safety
margin for future table zone expansions.

cockatrice/src/game/z_value_layer_manager.h

@@ -0,0 +1,136 @@
+/**
+ * @file z_value_layer_manager.h
+ * @brief Semantic Z-value layer management for game scene rendering.
+ *
+ * This file provides a structured approach to Z-value allocation in the game scene.
+ * Z-values in Qt determine stacking order - higher values render on top of lower values.
+ *
+ * ## Layer Architecture
+ *
+ * The game scene is organized into three conceptual layers:
+ *
+ * 1. **Zone Layer (0-999)**: Zone backgrounds, containers, and static elements
+ *    - Zone backgrounds (0.5-1.0)
+ *    - Cards within zones (1.0 base + index)
+ *
+ * 2. **Card Layer (1-40,000,000)**: Dynamic card rendering on the table zone
+ *    - Cards use formula: (actualY + CARD_HEIGHT) * 100000 + (actualX + 1) * 100
+ *    - Maximum card Z-value: ~40,000,000 (with 3 rows, actualY <= ~289)
+ *
+ * 3. **Overlay Layer (200,000,000+)**: UI elements that must appear above all cards
+ *    - Hovered cards (+1)
+ *    - Tax counters (+2)
+ *    - Arrows (+3)
+ *    - Zone views (+4)
+ *    - Drag items (+5, +6)
+ *    - Top UI elements (+7)
+ *
+ * ## Design Rationale
+ *
+ * The large gap between card Z-values (max ~40M) and overlay base (200M) provides
+ * safety margin for future table zone expansions while ensuring overlays always
+ * render above cards regardless of table position.
+ *
+ * ## Usage
+ *
+ * Prefer using the semantic constants from ZValues namespace:
+ * @code
+ * card->setZValue(ZValues::HOVERED_CARD);
+ * counter->setZValue(ZValues::TAX_COUNTERS);
+ * @endcode
+ *
+ * Use validation functions to verify card Z-values during development:
+ * @code
+ * Q_ASSERT(ZValueLayerManager::isValidCardZValue(cardZ));
+ * @endcode
+ */
+
+#ifndef Z_VALUE_LAYER_MANAGER_H
+#define Z_VALUE_LAYER_MANAGER_H
+
+#include <QtGlobal>
+
+/**
+ * @namespace ZValueLayerManager
+ * @brief Utilities for Z-value validation and layer management.
+ */
+namespace ZValueLayerManager
+{
+
+/**
+ * @enum Layer
+ * @brief Semantic layer identifiers for Z-value allocation.
+ *
+ * These represent conceptual rendering layers, not actual Z-values.
+ * Use the corresponding ZValues constants for actual rendering.
+ */
+enum class Layer
+{
+    /// Zone-level elements like backgrounds and containers
+    Zone,
+    /// Cards rendered in zones (uses sequential Z-values)
+    Card,
+    /// Temporary UI elements like hovered cards and drag items
+    Overlay
+};
+
+/**
+ * @brief Maximum Z-value a card can have on the table zone.
+ *
+ * Based on table zone formula: (actualY + CARD_HEIGHT) * 100000 + (actualX + 1) * 100
+ * With maximum 3 rows and CARD_HEIGHT ~96, actualY <= ~289.
+ * Maximum: (289 + 96) * 100000 + 100 * 100 = 38,510,000
+ *
+ * We use 40,000,000 as a safe upper bound with margin.
+ */
+constexpr qreal CARD_Z_VALUE_MAX = 40000000.0;
+
+/**
+ * @brief Base Z-value for overlay elements.
+ *
+ * Must exceed CARD_Z_VALUE_MAX to ensure overlays render above all cards.
+ * The 5x margin (200M vs 40M) provides safety for future expansion.
+ */
+constexpr qreal OVERLAY_BASE = 200000000.0;
+
+/**
+ * @brief Validates that a Z-value is within the valid card range.
+ *
+ * Cards should have Z-values between CARD_BASE (1.0) and CARD_Z_VALUE_MAX.
+ * Values outside this range may interfere with overlay rendering.
+ *
+ * @param zValue The Z-value to validate
+ * @return true if the Z-value is valid for a card
+ */
+[[nodiscard]] inline constexpr bool isValidCardZValue(qreal zValue)
+{
+    return zValue >= 1.0 && zValue <= CARD_Z_VALUE_MAX;
+}
+
+/**
+ * @brief Validates that a Z-value is in the overlay layer.
+ *
+ * Overlay elements should have Z-values at or above OVERLAY_BASE.
+ *
+ * @param zValue The Z-value to validate
+ * @return true if the Z-value is valid for an overlay element
+ */
+[[nodiscard]] inline constexpr bool isOverlayZValue(qreal zValue)
+{
+    return zValue >= OVERLAY_BASE;
+}
+
+/**
+ * @brief Returns the Z-value for a specific overlay element.
+ *
+ * @param offset Offset from OVERLAY_BASE (0-7 for current elements)
+ * @return The absolute Z-value for the overlay element
+ */
+[[nodiscard]] inline constexpr qreal overlayZValue(qreal offset)
+{
+    return OVERLAY_BASE + offset;
+}
+
+} // namespace ZValueLayerManager
+
+#endif // Z_VALUE_LAYER_MANAGER_H

cockatrice/src/game/z_values.h

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+#ifndef Z_VALUES_H
+#define Z_VALUES_H
+
+#include "z_value_layer_manager.h"
+
+/**
+ * @file z_values.h
+ * @brief Centralized Z-value constants for rendering layer order.
+ *
+ * Z-values in Qt determine stacking order. Higher values render on top.
+ * These constants define the visual layering hierarchy for the game scene.
+ *
+ * ## Layer Architecture
+ *
+ * See z_value_layer_manager.h for detailed documentation on the three-layer
+ * architecture (Zone, Card, Overlay) and the rationale for Z-value choices.
+ *
+ * ## Quick Reference
+ *
+ * | Layer    | Z-Value Range    | Purpose                           |
+ * |----------|------------------|-----------------------------------|
+ * | Zone     | 0.5 - 1.0        | Zone backgrounds, containers      |
+ * | Card     | 1.0 - 40,000,000 | Cards on table (position-based)   |
+ * | Overlay  | 200,000,000+     | UI elements above all cards       |
+ */
+
+namespace ZValues
+{
+
+// Expose base for callers that need it
+constexpr qreal OVERLAY_BASE = ZValueLayerManager::OVERLAY_BASE;
+
+// Overlay layer Z-values for items that should appear above normal cards
+constexpr qreal HOVERED_CARD = ZValueLayerManager::overlayZValue(1.0);
+constexpr qreal ARROWS = ZValueLayerManager::overlayZValue(3.0);
+constexpr qreal ZONE_VIEW_WIDGET = ZValueLayerManager::overlayZValue(4.0);
+constexpr qreal DRAG_ITEM = ZValueLayerManager::overlayZValue(5.0);
+constexpr qreal DRAG_ITEM_CHILD = ZValueLayerManager::overlayZValue(6.0);
+constexpr qreal TOP_UI = ZValueLayerManager::overlayZValue(7.0);
+
+// Card layering (general architecture, not command-zone specific)
+constexpr qreal CARD_BASE = 1.0;
+constexpr qreal CARD_MAX = ZValueLayerManager::CARD_Z_VALUE_MAX;
+
+} // namespace ZValues
+
+#endif // Z_VALUES_H