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/Combat/SituationFlags.cs

@@ -0,0 +1,61 @@
+namespace Theriapolis.Core.Rules.Combat;
+
+/// <summary>
+/// Per-attack situation modifiers. Flags compose: a Sneak Attack with
+/// Advantage and Disadvantage (e.g. attacker prone, target shadowed)
+/// cancels to a normal roll per d20 rules.
+///
+/// Phase 5 M4 wires the basic six (Advantage/Disadvantage and the four
+/// resolver-time tags); class-feature flags like Reckless Attack come in
+/// M6 once the feature engine reads from this enum.
+/// </summary>
+[System.Flags]
+public enum SituationFlags : uint
+{
+    None              = 0,
+    Advantage         = 1u << 0,
+    Disadvantage      = 1u << 1,
+    /// <summary>Attacker is at long range — disadvantage on the roll per d20.</summary>
+    LongRange         = 1u << 2,
+    /// <summary>Attacker has reach + a melee weapon vs. a target that has cover.</summary>
+    HalfCover         = 1u << 3,
+    ThreeQuartersCover= 1u << 4,
+    /// <summary>Attacker meets the Sneak Attack precondition (advantage or ally adjacent).</summary>
+    SneakAttackEligible = 1u << 5,
+    /// <summary>Attacker is firing a ranged weapon at a target within 5 ft. — disadvantage.</summary>
+    RangedInMelee     = 1u << 6,
+}
+
+public static class SituationFlagsExtensions
+{
+    /// <summary>
+    /// True when the situation should roll the d20 with advantage. Per
+    /// d20 rules, advantage and disadvantage cancel exactly (no doubling).
+    /// </summary>
+    public static bool RollsAdvantage(this SituationFlags f)
+    {
+        bool adv = (f & SituationFlags.Advantage) != 0;
+        bool dis = (f & SituationFlags.Disadvantage) != 0
+                || (f & SituationFlags.LongRange) != 0
+                || (f & SituationFlags.RangedInMelee) != 0;
+        return adv && !dis;
+    }
+
+    /// <summary>True when the situation rolls with disadvantage (and no compensating advantage).</summary>
+    public static bool RollsDisadvantage(this SituationFlags f)
+    {
+        bool adv = (f & SituationFlags.Advantage) != 0;
+        bool dis = (f & SituationFlags.Disadvantage) != 0
+                || (f & SituationFlags.LongRange) != 0
+                || (f & SituationFlags.RangedInMelee) != 0;
+        return dis && !adv;
+    }
+
+    /// <summary>Cover modifier applied to AC: 0 / 2 / 5.</summary>
+    public static int CoverAcBonus(this SituationFlags f)
+    {
+        if ((f & SituationFlags.ThreeQuartersCover) != 0) return 5;
+        if ((f & SituationFlags.HalfCover)          != 0) return 2;
+        return 0;
+    }
+}