Fix MCP stdio logging and prevent WebSocket port drift

[japd-dev]

Summary

This PR fixes a few issues that made the local Figma write integration unreliable:

• route resource registry logging away from stdio so MCP JSON-RPC stays valid
• fail fast on WebSocket port conflicts instead of silently drifting to a fallback port
• align local plugin localhost/WebSocket development configuration
• document the fixed-port expectation for the plugin/server pair

Why

The plugin currently connects to a fixed localhost port in practice. If the server silently falls back to another port, the MCP client and Figma plugin can end up connected to different server instances.

Also, non-protocol stdout output breaks MCP clients that expect clean JSON-RPC over stdio.

Testing

• ran npm run build
• verified duplicate startup no longer falls back to 8766
• verified Claude Desktop restarted onto 8765
• verified Figma plugin reconnected successfully over ws://localhost:8765