Refactor/z value constants (#6651)

* feat(z-values): add centralized Z-value constants infrastructure

Magic numbers scattered across the codebase make Z-value layering
hard to understand and maintain. Centralizing them provides:
- Self-documenting layer hierarchy
- Validation utilities for development
- Single source of truth for Z-value ranges

Two-tier header design:
- z_value_layer_manager.h: Foundation with constants and validation
- z_values.h: User-facing namespace with semantic constants

* refactor(z-values): replace magic Z-value numbers with ZValues constants

Magic numbers like 2000000007 are impossible to understand without
context. Named constants (ZValues::DRAG_ITEM) are self-documenting.

This intentionally renumbers overlay Z-values to use a cleaner offset
sequence. The relative stacking order is preserved, which is what
matters for correct rendering.

Each consumer now includes z_values.h and uses semantic constants
instead of magic numbers.

* refactor(z-values): removed redundant inline with contexpr, Updated doc, magic numbers removed from TableZone.

cockatrice/src/game/z_value_layer_manager.h

@@ -0,0 +1,136 @@
+/**
+ * @file z_value_layer_manager.h
+ * @ingroup GameGraphics
+ * @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 (2,000,000,000+)**: UI elements that must appear above all cards
+ *    - Hovered cards (+1)
+ *    - 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 (2B) 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);
+ * arrow->setZValue(ZValues::ARROWS);
+ * @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 50x margin (2B vs 40M) provides safety for future expansion.
+ */
+constexpr qreal OVERLAY_BASE = 2000000000.0;
+
+/**
+ * @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]] 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]] 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]] constexpr qreal overlayZValue(qreal offset)
+{
+    return OVERLAY_BASE + offset;
+}
+
+} // namespace ZValueLayerManager
+
+#endif // Z_VALUE_LAYER_MANAGER_H