Update .gitignore to exclude TypeScript build info and add dist directory. Expand README with project overview, technology stack, prerequisites, and instructions for running and testing the application.

src/HSchool.Simulation/Components/NetworkId.cs

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+namespace HSchool.Simulation.Components;
+
+/// <summary>
+/// Stable replication id. Arch entity ids are recycled, so the client gets this
+/// monotonically increasing value instead.
+/// </summary>
+public struct NetworkId
+{
+    public uint Value;
+
+    public NetworkId(uint value) => Value = value;
+}

src/HSchool.Simulation/Components/PlayerControl.cs

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+using HSchool.Protocol;
+
+namespace HSchool.Simulation.Components;
+
+/// <summary>Marks an entity as driven by a connected client's input.</summary>
+public struct PlayerControl
+{
+    /// <summary>Network id of the owning connection.</summary>
+    public uint PlayerId;
+
+    /// <summary>Latest intent received from that connection.</summary>
+    public InputButtons Buttons;
+
+    /// <summary>Sequence number of that intent; reserved for prediction/reconciliation.</summary>
+    public uint LastInputSequence;
+
+    /// <summary>Movement speed in units per second.</summary>
+    public float Speed;
+}

src/HSchool.Simulation/Components/Position.cs

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+namespace HSchool.Simulation.Components;
+
+/// <summary>World-space position in simulation units.</summary>
+public struct Position
+{
+    public float X;
+    public float Y;
+
+    public Position(float x, float y)
+    {
+        X = x;
+        Y = y;
+    }
+}

src/HSchool.Simulation/Components/Renderable.cs

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+using HSchool.Protocol;
+
+namespace HSchool.Simulation.Components;
+
+/// <summary>Everything the client needs to draw the entity; replicated verbatim in snapshots.</summary>
+public struct Renderable
+{
+    public EntityKind Kind;
+    public float Radius;
+
+    /// <summary>Packed 0x00RRGGBB.</summary>
+    public uint Color;
+}

src/HSchool.Simulation/Components/Velocity.cs

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+namespace HSchool.Simulation.Components;
+
+/// <summary>Simulation units per second, integrated by <c>MovementSystem</c>.</summary>
+public struct Velocity
+{
+    public float X;
+    public float Y;
+
+    public Velocity(float x, float y)
+    {
+        X = x;
+        Y = y;
+    }
+}