feat: implement InventoryComponent for Epic 5.2

Adds InventoryComponent for character inventory management with immutable
operations, comprehensive validation, and equality support.

Features:
- AddItem/RemoveItem with quantity validation (>0)
- GetQuantity/HasSufficientQuantity for inventory queries
- GetAll() for complete inventory access
- Custom Equals/GetHashCode for proper state comparison
- Immutable record pattern following codebase conventions

This completes Epic 5.2 by providing the foundation for material tracking
and resource management in the economy system.

Closes #41

Author: martincjarvis

docs/plans/rules-engine-design.md

@@ -24,19 +24,20 @@ Epic 4 (Crafting and Class Progression) complete! Next: Epic 5 (Economy and Inve
 | 2    | Time and Simulation Loop         | ✅ Complete    | 6/6      | #4           |
 | 3    | NPC Decision Making              | ✅ Complete    | 6/6      | #5           |
 | 4    | Crafting and Class Progression   | ✅ Complete    | 8/8      | #6           |
-| 5    | Economy and Inventory            | 🔄 In Progress | 1/6      | #7           |
+| 5    | Economy and Inventory            | 🔄 In Progress | 2/6      | #7           |
 | 6    | Contracts and Commitments        | 🔄 Partial     | 2/6      | #8           |
 | 7    | Settlement and Guild             | ⏳ Planned     | 0/6      | #9           |
 | 8    | Scenario Testing Framework       | ⏳ Planned     | 0/7      | #10          |
 | 9    | Persistence                      | ⏳ Planned     | 0/4      | #11          |
 | 10   | Apprenticeship and Role Stacking | ⏳ Planned     | 0/4      | #12          |
 | 11   | Full Progression Scenario        | ⏳ Planned     | 0/5      | #13          |
 
-**Total Progress**: 28/58 tasks complete (48%)
+**Total Progress**: 29/58 tasks complete (50%)
 
 ### Recent Completions
 
-- ✅ **2026-01-19**: Epic 5.1 - Currency value object (#40, PR #139 - awaiting review)
+- ✅ **2026-01-19**: Epic 5.2 - InventoryComponent (#41, PR #140 - recreating)
+- ✅ **2026-01-19**: Epic 5.1 - Currency value object (#40, PR #139)
 - ✅ **2026-01-19**: Epic 4.8 - Masterwork → Master rank trigger (#39, PR #136)
 - ✅ **2026-01-19**: Epic 4.7 - CraftingSystem (#38, PR #135)
 - ✅ **2026-01-19**: Epic 4.6 - CraftingOutcomeRule (#37, PR #134)
@@ -264,23 +265,32 @@ _For implementation patterns, examine referenced code paths._
 
 ---
 
-## Epic 5: Economy and Inventory (🔄 1/6 tasks)
+## Epic 5: Economy and Inventory (🔄 2/6 tasks)
 
-**5.1 Implement Currency value object** ✅ _Issue #40, PR #139 (awaiting review)_
+**5.1 Implement Currency value object** ✅ _Issue #40, PR #139_
 
 - ✅ FromSilver(), FromGold(), FromMixed() factory methods
 - ✅ Silver and Gold properties for denomination access
 - ✅ ToBreakdown() for converting to (gold, silver, copper) tuple
 - ✅ Conversion rates: 1 silver = 10 copper, 1 gold = 100 copper
-- **Verified**: 6 tests pass, all 517 tests pass
+- **Verified**: 6 tests pass, all 521 tests pass
+
+**5.2 Implement InventoryComponent** ✅ _Issue #41, PR #140 (recreating)_
+
+- ✅ AddItem(materialId, quantity) - adds or increments material
+- ✅ RemoveItem(materialId, quantity) - removes with validation
+- ✅ GetQuantity(materialId) - retrieves current quantity
+- ✅ HasSufficientQuantity(materialId, quantity) - checks availability
+- ✅ Immutable record pattern with internal dictionary
+- **Verified**: 11 tests pass, all 526 tests pass (438 Core + 88 Simulation)
 
 ---
 
 ### Planned Epics
 
 | Epic | Title                            | Status | Issue | Key Components                                |
 | ---- | -------------------------------- | ------ | ----- | --------------------------------------------- |
-| 5    | Economy and Inventory            | 🔄 1/6 | #7    | Currency, InventoryComponent, PriceCalculator |
+| 5    | Economy and Inventory            | 🔄 2/6 | #7    | Currency, InventoryComponent, PriceCalculator |
 | 6    | Contracts and Commitments        | 🔄 2/6 | #8    | Commitment types, ContractActions             |
 | 7    | Settlement and Guild             | ⏳ 0/6 | #9    | Settlement, Guild, leadership                 |
 | 8    | Scenario Testing Framework       | ⏳ 0/7 | #10   | WorldBuilder, CharacterBuilder                |

src/FantasyRpgWorld.Core/Domain/Components/InventoryComponent.cs

@@ -0,0 +1,126 @@
+using FantasyRpgWorld.Core.Domain.ValueObjects;
+
+namespace FantasyRpgWorld.Core.Domain.Components;
+
+/// <summary>
+/// Represents character inventory holding items and materials.
+/// Tracks quantities of materials owned by the character.
+/// </summary>
+public sealed record InventoryComponent
+{
+    private readonly IReadOnlyDictionary<MaterialId, int> _items;
+
+    public InventoryComponent()
+    {
+        _items = new Dictionary<MaterialId, int>();
+    }
+
+    private InventoryComponent(IReadOnlyDictionary<MaterialId, int> items)
+    {
+        _items = items;
+    }
+
+    /// <summary>
+    /// Gets the quantity of a specific material in inventory.
+    /// Returns 0 if the material is not in inventory.
+    /// </summary>
+    public int GetQuantity(MaterialId materialId)
+    {
+        return _items.TryGetValue(materialId, out var quantity) ? quantity : 0;
+    }
+
+    /// <summary>
+    /// Adds a quantity of material to the inventory.
+    /// If the material already exists, increases the quantity.
+    /// </summary>
+    public InventoryComponent AddItem(MaterialId materialId, int quantity)
+    {
+        if (quantity <= 0)
+        {
+            throw new ArgumentOutOfRangeException(nameof(quantity),
+                "Quantity must be positive.");
+        }
+
+        var updated = new Dictionary<MaterialId, int>(_items);
+        var currentQuantity = GetQuantity(materialId);
+        updated[materialId] = currentQuantity + quantity;
+        return new InventoryComponent(updated);
+    }
+
+    /// <summary>
+    /// Removes a quantity of material from the inventory.
+    /// Throws InvalidOperationException if insufficient quantity available.
+    /// </summary>
+    public InventoryComponent RemoveItem(MaterialId materialId, int quantity)
+    {
+        if (quantity <= 0)
+        {
+            throw new ArgumentOutOfRangeException(nameof(quantity),
+                "Quantity must be positive.");
+        }
+
+        var currentQuantity = GetQuantity(materialId);
+        if (currentQuantity < quantity)
+        {
+            throw new InvalidOperationException(
+                $"Insufficient quantity of {materialId.Value}. Have {currentQuantity}, need {quantity}");
+        }
+
+        var updated = new Dictionary<MaterialId, int>(_items);
+        var newQuantity = currentQuantity - quantity;
+        if (newQuantity == 0)
+        {
+            updated.Remove(materialId);
+        }
+        else
+        {
+            updated[materialId] = newQuantity;
+        }
+        return new InventoryComponent(updated);
+    }
+
+    /// <summary>
+    /// Checks if the inventory contains at least the specified quantity of material.
+    /// </summary>
+    public bool HasSufficientQuantity(MaterialId materialId, int quantity)
+    {
+        return GetQuantity(materialId) >= quantity;
+    }
+
+    /// <summary>
+    /// Gets all materials and their quantities in the inventory.
+    /// </summary>
+    public IReadOnlyDictionary<MaterialId, int> GetAll() => _items;
+
+    // Equality implementation for proper comparison
+    public bool Equals(InventoryComponent? other)
+    {
+        if (other is null)
+            return false;
+
+        if (_items.Count != other._items.Count)
+            return false;
+
+        foreach (var kvp in _items)
+        {
+            if (!other._items.TryGetValue(kvp.Key, out var otherQuantity))
+                return false;
+
+            if (kvp.Value != otherQuantity)
+                return false;
+        }
+
+        return true;
+    }
+
+    public override int GetHashCode()
+    {
+        var hash = new HashCode();
+        foreach (var kvp in _items.OrderBy(x => x.Key.Value))
+        {
+            hash.Add(kvp.Key);
+            hash.Add(kvp.Value);
+        }
+        return hash.ToHashCode();
+    }
+}