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/Data/BackgroundDef.cs

@@ -0,0 +1,36 @@
+using System.Text.Json.Serialization;
+
+namespace Theriapolis.Core.Data;
+
+/// <summary>
+/// Immutable background definition loaded from backgrounds.json.
+/// Phase 5 grants the listed skill / tool proficiencies but does not
+/// apply the named feature's mechanical effect — those resolve to
+/// dialogue / quest / faction systems shipped in Phase 6.
+/// </summary>
+public sealed record BackgroundDef
+{
+    [JsonPropertyName("id")]
+    public string Id { get; init; } = "";
+
+    [JsonPropertyName("name")]
+    public string Name { get; init; } = "";
+
+    [JsonPropertyName("flavor")]
+    public string Flavor { get; init; } = "";
+
+    [JsonPropertyName("skill_proficiencies")]
+    public string[] SkillProficiencies { get; init; } = Array.Empty<string>();
+
+    [JsonPropertyName("tool_proficiencies")]
+    public string[] ToolProficiencies { get; init; } = Array.Empty<string>();
+
+    [JsonPropertyName("feature_name")]
+    public string FeatureName { get; init; } = "";
+
+    [JsonPropertyName("feature_description")]
+    public string FeatureDescription { get; init; } = "";
+
+    [JsonPropertyName("suggested_personality")]
+    public string SuggestedPersonality { get; init; } = "";
+}