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/EncounterTrigger.cs

@@ -0,0 +1,82 @@
+using Theriapolis.Core.Entities;
+using Theriapolis.Core.Util;
+
+namespace Theriapolis.Core.Rules.Combat;
+
+/// <summary>
+/// Per-tick check used by <see cref="Game.Screens.PlayScreen"/>:
+/// "is there a hostile NPC within encounter trigger range that has line of
+/// sight?" Returns the closest qualifying actor (or null) so the caller can
+/// kick off an encounter.
+///
+/// Friendly / Neutral proximity is the same shape but uses a tighter radius
+/// — see <see cref="FindInteractCandidate"/>.
+/// </summary>
+public static class EncounterTrigger
+{
+    /// <summary>
+    /// Find the closest live <em>hostile</em> NPC within
+    /// <see cref="C.ENCOUNTER_TRIGGER_TILES"/> of the player that the
+    /// <paramref name="losBlocked"/> predicate can see (no blocking tile
+    /// between). Returns null if none found.
+    /// </summary>
+    public static NpcActor? FindHostileTrigger(
+        ActorManager actors,
+        System.Func<int, int, bool>? losBlocked = null)
+    {
+        var player = actors.Player;
+        if (player is null) return null;
+
+        var blocked = losBlocked ?? LineOfSight.AlwaysClear;
+        NpcActor? best = null;
+        int bestDistSq = C.ENCOUNTER_TRIGGER_TILES * C.ENCOUNTER_TRIGGER_TILES;
+
+        foreach (var npc in actors.Npcs)
+        {
+            if (!npc.IsAlive) continue;
+            if (npc.Allegiance != Rules.Character.Allegiance.Hostile) continue;
+            int distSq = ChebyshevDistSq(player.Position, npc.Position);
+            if (distSq > bestDistSq) continue;
+            if (!LineOfSight.HasLine(player.Position, npc.Position, blocked)) continue;
+            if (best is null || distSq < bestDistSq) { best = npc; bestDistSq = distSq; }
+        }
+        return best;
+    }
+
+    /// <summary>
+    /// Friendly / Neutral NPCs within
+    /// <see cref="C.INTERACT_PROMPT_TILES"/> of the player. The HUD shows
+    /// "[F] Talk to ..." for the closest match.
+    /// </summary>
+    public static NpcActor? FindInteractCandidate(
+        ActorManager actors,
+        System.Func<int, int, bool>? losBlocked = null)
+    {
+        var player = actors.Player;
+        if (player is null) return null;
+
+        var blocked = losBlocked ?? LineOfSight.AlwaysClear;
+        NpcActor? best = null;
+        int bestDistSq = C.INTERACT_PROMPT_TILES * C.INTERACT_PROMPT_TILES;
+
+        foreach (var npc in actors.Npcs)
+        {
+            if (!npc.IsAlive) continue;
+            if (npc.Allegiance != Rules.Character.Allegiance.Friendly &&
+                npc.Allegiance != Rules.Character.Allegiance.Neutral) continue;
+            int distSq = ChebyshevDistSq(player.Position, npc.Position);
+            if (distSq > bestDistSq) continue;
+            if (!LineOfSight.HasLine(player.Position, npc.Position, blocked)) continue;
+            if (best is null || distSq < bestDistSq) { best = npc; bestDistSq = distSq; }
+        }
+        return best;
+    }
+
+    private static int ChebyshevDistSq(Vec2 a, Vec2 b)
+    {
+        int dx = (int)System.Math.Abs(a.X - b.X);
+        int dy = (int)System.Math.Abs(a.Y - b.Y);
+        int d  = System.Math.Max(dx, dy);
+        return d * d;
+    }
+}