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/Tactical/IChunkDeltaStore.cs

@@ -0,0 +1,57 @@
+namespace Theriapolis.Core.Tactical;
+
+/// <summary>
+/// Persists per-chunk player-caused modifications (chopped trees, dropped items,
+/// emptied bandit camps). Implementations: in-memory (Phase 4 tests + runtime
+/// cache), MessagePack-backed (Phase 4 save body — see <see cref="ChunkDelta"/>).
+/// </summary>
+public interface IChunkDeltaStore
+{
+    /// <summary>Returns the delta for a chunk, or null if no modifications recorded.</summary>
+    ChunkDelta? Get(ChunkCoord cc);
+
+    /// <summary>Records a chunk's delta. Overwrites any existing entry.</summary>
+    void Put(ChunkCoord cc, ChunkDelta delta);
+
+    /// <summary>Erases a chunk's delta — the chunk reverts to its deterministic baseline.</summary>
+    void Remove(ChunkCoord cc);
+
+    /// <summary>Snapshot of every recorded delta, used by SaveCodec to flush the world.</summary>
+    IReadOnlyDictionary<ChunkCoord, ChunkDelta> All { get; }
+}
+
+/// <summary>
+/// Sparse delta for a single chunk: just the tactical-tile cells that diverge
+/// from <see cref="TacticalChunkGen.Generate"/>'s output. Phase 4 stores both
+/// the surface and the deco; richer per-tile state arrives later.
+/// </summary>
+public sealed class ChunkDelta
+{
+    public List<TileMod> TileMods { get; } = new();
+
+    /// <summary>True if the spawn list has been consumed (all spawns cleared).</summary>
+    public bool SpawnsConsumed { get; set; }
+}
+
+public readonly struct TileMod
+{
+    public readonly byte LocalX;
+    public readonly byte LocalY;
+    public readonly TacticalSurface Surface;
+    public readonly TacticalDeco    Deco;
+    public readonly byte            Flags;
+    public TileMod(int lx, int ly, TacticalSurface s, TacticalDeco d, byte f)
+    {
+        LocalX = (byte)lx; LocalY = (byte)ly; Surface = s; Deco = d; Flags = f;
+    }
+}
+
+/// <summary>Trivial in-memory implementation. Used by tests and at runtime before save.</summary>
+public sealed class InMemoryChunkDeltaStore : IChunkDeltaStore
+{
+    private readonly Dictionary<ChunkCoord, ChunkDelta> _store = new();
+    public ChunkDelta? Get(ChunkCoord cc) => _store.TryGetValue(cc, out var d) ? d : null;
+    public void Put(ChunkCoord cc, ChunkDelta delta) => _store[cc] = delta;
+    public void Remove(ChunkCoord cc) => _store.Remove(cc);
+    public IReadOnlyDictionary<ChunkCoord, ChunkDelta> All => _store;
+}