Initial commit: Theriapolis baseline at port/godot branch point

Captures the pre-Godot-port state of the codebase. This is the rollback
anchor for the Godot port (M0 of theriapolis-rpg-implementation-plan-godot-port.md).
All Phase 0 through Phase 6.5 work is included; Phase 7 is in flight.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Theriapolis.Core/Items/ItemInstance.cs

@@ -0,0 +1,38 @@
+using Theriapolis.Core.Data;
+
+namespace Theriapolis.Core.Items;
+
+/// <summary>
+/// One stack of items in an <see cref="Inventory"/>. Holds a reference to the
+/// immutable <see cref="ItemDef"/> plus per-instance state: how many in the
+/// stack, current condition, and (optionally) the slot the item is equipped
+/// into.
+///
+/// Phase 5 ships condition as a no-op (always 100); it's reserved for damage,
+/// repairs, and weapon breaking that arrive in Phase 5.5+.
+/// </summary>
+public sealed class ItemInstance
+{
+    public ItemDef Def { get; }
+    public int     Qty { get; set; }
+
+    /// <summary>Condition, 0..100. 100 = pristine. Phase 5 always uses 100.</summary>
+    public int Condition { get; set; } = 100;
+
+    /// <summary>Null while in the inventory bag; set when the item is equipped.</summary>
+    public EquipSlot? EquippedAt { get; set; }
+
+    public ItemInstance(ItemDef def, int qty = 1)
+    {
+        Def = def ?? throw new ArgumentNullException(nameof(def));
+        if (qty < 1) throw new ArgumentOutOfRangeException(nameof(qty), "qty must be ≥ 1");
+        Qty = qty;
+    }
+
+    public float TotalWeightLb => Def.WeightLb * Qty;
+
+    public override string ToString() =>
+        EquippedAt is null
+            ? $"{Def.Name}{(Qty > 1 ? $" ×{Qty}" : "")}"
+            : $"{Def.Name} (equipped: {EquippedAt})";
+}