replay: add determinism foundation and double-run verification (Phase 0)

Introduces the state-checksum machinery the replay feature rests on, and a
test that proves the simulation is deterministic for a given seed/binary.

- StateChecksum: FNV-1a Hasher (bit-pattern float hashing, -0 normalization,
  length-tagged strings) plus a portable mt19937 state fingerprint.
- BeltSystem/BuildingSystem: appendChecksum(Hasher&) folding transport and
  building/site/occupancy state in deterministic (sorted/insertion) order.
- Simulation: rngFingerprint() (cheap, for the future file checksum) and
  computeStateChecksum() (full state: RNG, scalars, wave/schematic/unlock
  state, subsystems, and ECS position/health/facing/body/scrap/identity).
- DeterminismTest: Hasher unit tests, RNG-fingerprint tests, and a double-run
  test asserting identical per-tick full-state checksums from one seed; plus a
  different-seed divergence guard.

Exit criteria met: double-run determinism test passes; full suite green
(334 cases / 3346 assertions).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DUsFgd2Ga6pmLz8giS8WUn

src/lib/sim/Simulation.h

@@ -24,6 +24,7 @@
 
 class AiSystem;
 class BuildingSystem;
+class Hasher;
 class CombatSystem;
 class DynamicBodySystem;
 class MovementIntentSystem;
@@ -88,6 +89,16 @@ public:
     bool isRecipeUnlocked(const std::string& recipeId) const;
     bool isItemUnlocked(const std::string& itemId) const;
 
+    // -- Determinism (see docs/replay_design.md) -----------------------------
+    // 64-bit fingerprint of the RNG stream state. Cheap; written to the replay
+    // file periodically + after each command for desync detection.
+    unsigned long long rngFingerprint() const;
+
+    // 64-bit fingerprint of the full simulation state (RNG, scalars, buildings,
+    // belts, and ECS component state). Used by the double-run determinism test;
+    // a superset of rngFingerprint().
+    unsigned long long computeStateChecksum() const;
+
     // Checks affordability, deducts building blocks, and places the building.
     // Returns the new entity id, or kInvalidBuildingId if blocks are insufficient.
     BuildingId tryPlaceBuilding(BuildingType type, QPoint anchor, Rotation rotation);
@@ -149,6 +160,11 @@ private:
     std::map<std::string, SchematicState> m_schematicLevels;
     std::map<std::string, SchematicState> m_moduleSchematicLevels;
 
+    // Determinism helpers — fold sub-state into the hasher in deterministic order.
+    static void appendSchematicMap(Hasher& hasher,
+                                   const std::map<std::string, SchematicState>& levels);
+    static void appendStringSet(Hasher& hasher, const std::set<std::string>& ids);
+
     // Explicitly unlocked assembler recipe schematics (REQ-LOCK-EXPLICIT).
     std::set<std::string> m_unlockedRecipeSchematicIds;