Replay: deterministic record & playback (#4)

Add deterministic record/playback for a run.

Recording captures `(seed, config hash, ordered tick-tagged commands)` and re-simulates on playback — no state snapshots. `DotaFactory.exe --replay <file>` re-plays a recorded run view-only with manual speed/pause.

Reviewed-on: #4
Co-authored-by: Malte Langkabel <malte.langkabel@gmail.com>
Co-committed-by: Malte Langkabel <malte.langkabel@gmail.com>

src/lib/sim/BeltSystem.h

@@ -15,6 +15,8 @@
 #include "Port.h"
 #include "Rotation.h"
 
+class Hasher;
+
 // Carries item type and fractional world position for the renderer.
 // worldPos is in tile units (1 tile = 1.0 unit); origin matches tile coords.
 struct VisualItem
@@ -92,6 +94,11 @@ public:
     void forEachVisualItem(QRect viewportTiles,
                            std::function<void(VisualItem)> visit) const;
 
+    // -- Determinism ---------------------------------------------------------
+    // Folds all transport state (belt/splitter/tunnel tiles and their items)
+    // into the hasher in deterministic order (see docs/replay_design.md).
+    void appendChecksum(Hasher& hasher) const;
+
 private:
     void advanceProgress();
     void advanceTunnelProgress();
@@ -170,6 +177,9 @@ private:
         std::vector<TunnelTransitItem> items;  // front (highest progress) to back
     };
 
+    // Folds a run of item slots (front-to-back order is canonical) into the hasher.
+    static void appendItemSlots(Hasher& hasher, const std::vector<BeltItemSlot>& slotRun);
+
     double m_progressPerTick_tpt;  // beltSpeed_tps / kTickRateHz
 
     std::map<std::pair<int, int>, BeltTile>        m_belts;