Implement deterministic simulation spine

src/SideScrollerGame.Sim/Serialization/SimulationStateSerializer.cs

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+using System.Collections.Immutable;
+using System.Diagnostics.CodeAnalysis;
+using System.Text.Json;
+using SideScrollerGame.Sim.Math;
+using SideScrollerGame.Sim.Runtime;
+
+namespace SideScrollerGame.Sim.Serialization;
+
+[ExcludeFromCodeCoverage]
+internal static class SimulationStateSerializer
+{
+    [ExcludeFromCodeCoverage]
+    private sealed record SimulationStateDocument
+    {
+        public int Version { get; init; }
+
+        public int Tick { get; init; }
+
+        public int Seed { get; init; }
+
+        public ulong RandomState { get; init; }
+
+        public ulong LastRandomValue { get; init; }
+
+        public ImmutableArray<PlayerStateDocument> Players { get; init; }
+    }
+
+    [ExcludeFromCodeCoverage]
+    private sealed record PlayerStateDocument
+    {
+        public int PlayerId { get; init; }
+
+        public int PositionX { get; init; }
+
+        public int PositionY { get; init; }
+
+        public sbyte MoveAxisX { get; init; }
+
+        public sbyte MoveAxisY { get; init; }
+
+        public short AimAxisX { get; init; }
+
+        public short AimAxisY { get; init; }
+
+        public int SelectedWeaponSlot { get; init; }
+
+        public int ButtonMask { get; init; }
+    }
+
+    public static byte[] Serialize(SimulationState state)
+    {
+        List<PlayerStateDocument> players = new(state.Players.Length);
+        foreach (var player in state.Players)
+        {
+            players.Add(new()
+            {
+                PlayerId = player.PlayerId.Value,
+                PositionX = player.Position.m_X.m_Value,
+                PositionY = player.Position.m_Y.m_Value,
+                MoveAxisX = player.MoveAxisX,
+                MoveAxisY = player.MoveAxisY,
+                AimAxisX = player.AimAxisX,
+                AimAxisY = player.AimAxisY,
+                SelectedWeaponSlot = player.SelectedWeaponSlot,
+                ButtonMask = player.ButtonMask
+            });
+        }
+
+        return JsonSerializer.SerializeToUtf8Bytes(new SimulationStateDocument
+        {
+            Version = SimulationDefaults.StateFormatVersion,
+            Tick = state.Tick,
+            Seed = state.Seed,
+            RandomState = state.RandomState,
+            LastRandomValue = state.LastRandomValue,
+            Players = players.ToImmutableArray()
+        });
+    }
+
+    public static SimulationState Deserialize(byte[] data)
+    {
+        var document = JsonSerializer.Deserialize<SimulationStateDocument>(data) ?? throw new InvalidOperationException("Simulation state payload was empty.");
+        if (document.Version != SimulationDefaults.StateFormatVersion)
+            throw new NotSupportedException($"Unsupported simulation state version {document.Version}.");
+
+        var players = ImmutableArray.CreateBuilder<PlayerState>(document.Players.Length);
+        foreach (var player in document.Players)
+        {
+            players.Add(new(new(player.PlayerId), new(new() { m_Value = player.PositionX }, new FixPoint16 { m_Value = player.PositionY }), player.MoveAxisX, player.MoveAxisY, player.AimAxisX, player.AimAxisY, player.SelectedWeaponSlot, player.ButtonMask));
+        }
+
+        return new(document.Tick, document.Seed, document.RandomState, document.LastRandomValue, players.MoveToImmutable());
+    }
+}