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/ProficiencyBonus.cs

@@ -0,0 +1,32 @@
+namespace Theriapolis.Core.Rules.Stats;
+
+/// <summary>
+/// Standard d20 proficiency-bonus-by-level table:
+///   1-4   → +2
+///   5-8   → +3
+///   9-12  → +4
+///   13-16 → +5
+///   17-20 → +6
+/// Phase 5 only ever evaluates level 1, but the full table ships so
+/// future leveling work doesn't have to revisit this file.
+/// </summary>
+public static class ProficiencyBonus
+{
+    public const int MinLevel = 1;
+    public const int MaxLevel = 20;
+
+    public static int ForLevel(int level)
+    {
+        if (level < MinLevel || level > MaxLevel)
+            throw new ArgumentOutOfRangeException(nameof(level), $"Level must be {MinLevel}..{MaxLevel}, got {level}");
+
+        return level switch
+        {
+            >= 17 => 6,
+            >= 13 => 5,
+            >=  9 => 4,
+            >=  5 => 3,
+            _     => 2,
+        };
+    }
+}