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/Rules/Stats/SaveId.cs

@@ -0,0 +1,32 @@
+namespace Theriapolis.Core.Rules.Stats;
+
+/// <summary>
+/// Saving-throw categories. There's exactly one per ability; this enum is
+/// a thin alias of <see cref="AbilityId"/> kept distinct so callsites read
+/// clearly ("MakeSave(SaveId.DEX, dc)" vs "Mod(AbilityId.DEX)").
+/// </summary>
+public enum SaveId : byte
+{
+    STR = 0,
+    DEX = 1,
+    CON = 2,
+    INT = 3,
+    WIS = 4,
+    CHA = 5,
+}
+
+public static class SaveIdExtensions
+{
+    public static AbilityId Ability(this SaveId s) => (AbilityId)(byte)s;
+
+    public static SaveId FromJson(string raw) => raw.ToUpperInvariant() switch
+    {
+        "STR" => SaveId.STR,
+        "DEX" => SaveId.DEX,
+        "CON" => SaveId.CON,
+        "INT" => SaveId.INT,
+        "WIS" => SaveId.WIS,
+        "CHA" => SaveId.CHA,
+        _ => throw new ArgumentException($"Unknown save: '{raw}'"),
+    };
+}