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/DeathSaveTracker.cs

@@ -0,0 +1,79 @@
+using Theriapolis.Core.Rules.Stats;
+
+namespace Theriapolis.Core.Rules.Combat;
+
+/// <summary>
+/// Phase 5 M6 player death-save loop. d20 every turn while at 0 HP:
+/// - 1            → 2 failures
+/// - 2..9         → 1 failure
+/// - 10..19       → 1 success
+/// - 20           → revive at 1 HP (zero out failures + successes)
+///
+/// 3 cumulative successes (≥10) → stabilised at 0 HP (cleared on heal).
+/// 3 cumulative failures (<10)  → dead. CombatHUDScreen pushes
+/// <see cref="Game.Screens.DefeatedScreen"/> when this fires.
+///
+/// Tracker lives on <see cref="Combatant"/> only for the player; NPC
+/// combatants skip death saves and are removed at 0 HP.
+/// </summary>
+public sealed class DeathSaveTracker
+{
+    public int Successes { get; private set; }
+    public int Failures  { get; private set; }
+    public bool Stabilised { get; private set; }
+    public bool Dead       { get; private set; }
+
+    /// <summary>Roll a death save and update counters. Returns the outcome.</summary>
+    public DeathSaveOutcome Roll(Encounter enc, Combatant target)
+    {
+        if (Dead || Stabilised) return DeathSaveOutcome.NoOp;
+
+        int d20 = enc.RollD20();
+        DeathSaveOutcome outcome;
+        if (d20 == 20)
+        {
+            // Critical success — revive at 1 HP.
+            target.CurrentHp = 1;
+            target.Conditions.Remove(Condition.Unconscious);
+            Successes = 0;
+            Failures  = 0;
+            outcome   = DeathSaveOutcome.CriticalRevive;
+        }
+        else if (d20 >= 10)
+        {
+            Successes++;
+            outcome = Successes >= 3 ? DeathSaveOutcome.Stabilised : DeathSaveOutcome.Success;
+            if (Successes >= 3) Stabilised = true;
+        }
+        else
+        {
+            int failsThisRoll = d20 == 1 ? 2 : 1;
+            Failures += failsThisRoll;
+            outcome   = Failures >= 3 ? DeathSaveOutcome.Dead : DeathSaveOutcome.Failure;
+            if (Failures >= 3) Dead = true;
+        }
+
+        enc.AppendLog(CombatLogEntry.Kind.Save,
+            $"{target.Name} death save: {d20} → {outcome} ({Successes}S/{Failures}F)");
+        return outcome;
+    }
+
+    /// <summary>Called when the character is healed above 0 HP — cancels the loop.</summary>
+    public void Reset()
+    {
+        Successes = 0;
+        Failures  = 0;
+        Stabilised = false;
+        // Don't reset Dead — once dead, stays dead.
+    }
+}
+
+public enum DeathSaveOutcome
+{
+    NoOp           = 0,
+    Success        = 1,
+    Failure        = 2,
+    Stabilised     = 3,
+    Dead           = 4,
+    CriticalRevive = 5,
+}