Add MrGameEng.Net: WebSocket transport + delta component replication

Browsers can't speak UDP, so WebSocket is the engine's one transport.
The server side is a dependency-free RFC 6455 implementation over
TcpListener (handshake, frame codec with masking and fragmentation,
ping/pong — unit-tested against the RFC example vectors); the client
wraps ClientWebSocket, which works on desktop and maps to the browser
WebSocket in Blazor WASM. Both sit behind the poll-based INetConnection
so simulation systems drain messages from their own thread; client
sends are chained fire-and-forget (no blocking — wasm-safe).

Replication is server-authoritative: games register unmanaged component
types in a ReplicationSchema (same order both sides, up to 32 types),
ReplicationServer snapshots entities carrying NetId once per send and
ships each connection only the components that changed since its last
snapshot — a reliable ordered transport needs no acks for deltas. New
connections receive the full state through the same path; despawns are
tracked by set difference. ReplicationClient applies snapshots to a
local EntityStore and raises EntitySpawned so the game can decorate
replicated entities with presentation components.

Covered by 16 tests including a real loopback exchange between
WebSocketClient and WebSocketServer.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

CLAUDE.md

@@ -64,6 +64,13 @@ Engine libraries (each feature is a namespaced subfolder of its host):
   **Collisions** (`MrGameEng.Collisions`: `Collider` component, spatial hash rebuilt per
   tick, pairs/queries/raycast, `scene.UseCollisions()` after movement systems).
   → `Core`, `Graphics` (Collisions needs `Transform2D`, `RectF`).
+- **`Net`** — multiplayer building blocks, browser-compatible by design: a dependency-free
+  RFC 6455 WebSocket server over `TcpListener` (browsers can't speak UDP, so WebSocket is
+  the engine's one transport), a `ClientWebSocket`-based client (works in Blazor WASM),
+  both behind the poll-based `INetConnection`; server-authoritative component replication
+  (`ReplicationSchema` of unmanaged components, `ReplicationServer` sending per-connection
+  deltas — no acks needed over a reliable ordered transport, `ReplicationClient` applying
+  snapshots to a local store, `NetId`). → `Core`.
 - **`UI`** — Myra integration (`scene.UseUI()` after `UseRenderer2D()`),
   **DevConsole** (`MrGameEng.DevConsole`: in-game console capturing `Core.Log`,
   `scene.UseDevConsole()` last in OnLoad) and **Inspector** (`MrGameEng.Inspector`: a