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/Dungeons/RoomConnection.cs

@@ -0,0 +1,23 @@
+namespace Theriapolis.Core.Dungeons;
+
+/// <summary>
+/// Phase 7 M1 — a connection between two rooms in a dungeon, anchored by
+/// the door tile on each side. Connections are created by
+/// <see cref="RoomGraphAssembler"/> when matching one room's outgoing door
+/// to another's incoming door; they're authoritative for reachability
+/// (BFS over connections, not over the painted tile array).
+///
+/// Door coordinates are in dungeon-local tile space and address the
+/// *door tile itself* (carved out of the perimeter wall). The door state
+/// (open / closed / locked) lives in <see cref="Theriapolis.Core.Persistence.DungeonStateSnapshot"/>
+/// — this record carries only the deterministic baseline.
+/// </summary>
+public readonly record struct RoomConnection(
+    int RoomA,
+    int DoorAx,
+    int DoorAy,
+    int RoomB,
+    int DoorBx,
+    int DoorBy,
+    /// <summary>Lock difficulty tier or "" for unlocked. Matches RoomDoor.Lock values.</summary>
+    string Lock = "");