feat: add built-in MCP server

[LibraHp]

Summary

This PR adds a native MCP server directly inside CinematicUnityExplorer.

The MCP server is implemented entirely in C# and runs inside the game process. It does not require Node.js, an
external bridge process, or any additional runtime dependency.

The implementation supports both legacy SSE and Streamable HTTP transports, selectable from the in-game MCP settings
panel.

Preview

Changes

Built-in MCP server

• Add a native MCP server under src/MCP/.
• Implement JSON-RPC request handling inside the DLL.
• Add support for:
  • initialize
  • notifications/initialized
  • ping
  • tools/list
  • tools/call
• Add Unity main-thread request queuing and execution.
• Ensure Unity object access and mutations are performed from the Unity update loop.

Runtime game-control tools

Add MCP tools for:

• Querying runtime status.
• Listing loaded scenes.
• Searching and inspecting GameObjects.
• Reading fields, properties, and nested member paths.
• Discovering instance and static methods.
• Modifying fields and properties.
• Modifying transforms.
• Enabling and disabling GameObjects or components.
• Invoking methods with converted arguments.
• Creating GameObjects, primitives, and components.
• Destroying GameObjects.
• Executing multiple operations in sequence.

IL2CPP compatibility

• Add runtime type resolution for IL2CPP-generated wrapper types.
• Support accessing fields and methods declared on IL2CPP-derived game classes instead of only the exposed base
  wrapper type.
• Add value conversion between JSON values and CLR/Unity/IL2CPP values.
• Avoid passing System.Collections.Generic.List<T> into IL2CPP Unity APIs where
  Il2CppSystem.Collections.Generic.List<T> is required.

HTTP transports

• Add legacy SSE transport support.
• Add Streamable HTTP transport support.
• Allow switching transport mode from the MCP UI.
• Keep the server bound to the loopback address by default.
• Add bearer-token authentication.
• Add request size and queue limits.
• Add configurable request timeout and per-frame execution limits.

MCP settings UI

Add a scrollable MCP settings panel with the same visual style as the other CUE panels.

The panel provides controls for:

• Enable/disable MCP.
• Transport mode.
• Bind address.
• Port.
• RPC path.
• Health-check path.
• Authentication token.
• Health endpoint authentication.
• Read-only mode.
• Dangerous-operation permission.
• Request logging.
• Request timeout.
• Maximum request body size.
• Maximum pending requests.
• Maximum requests executed per frame.

The UI also displays runtime status and generates client configuration information for Agent/MCP clients.

Documentation

Add:

• docs/MCP.md
• docs/MCP_SECURITY.md

The documentation describes:

• DLL-only deployment.
• SSE and Streamable HTTP configuration.
• Trae configuration.
• Available MCP tools.
• Read-only and dangerous-operation modes.
• Authentication requirements.
• Runtime and prompt-injection security considerations.
• Object-handle lifetime limitations.

Security considerations

The MCP server is disabled by default and binds to 127.0.0.1 by default.

Mutation and dangerous operations are protected by runtime checks:

• Read-only mode blocks mutation operations.
• Dangerous operations require an additional explicit permission.
• Authentication is enforced inside the DLL.
• Batch operations apply permission checks to each sub-operation.
• Requests are queued before Unity main-thread execution.
• The server does not rely on MCP client-side approval settings for security.

Users should only enable write access for trusted local MCP clients and should use a recoverable test save.

Compatibility

The implementation is intended to support:

• Standalone Mono builds.
• Standalone IL2CPP builds using Unhollower.
• IL2CPP interop builds.

The MCP transport and runtime code does not depend on Node.js or an external process.

Testing

• Performed static source review of the Mono/IL2CPP Dropdown API differences.
• Verified that the transport-mode dropdown uses individual Dropdown.OptionData entries, which avoids the IL2CPP
  List<T> type mismatch.
• Verified that test scripts and the legacy mcp-server/ Node.js adapter are not part of this PR.
• Full project build was not run in this environment.
• Manual validation in a target Unity game is still recommended for:
  • SSE connection establishment.
  • Streamable HTTP requests.
  • Trae MCP configuration.
  • IL2CPP-derived field/property access.
  • Method invocation and mutation permissions.