Multi-client support for the Blender bridge.

- MCP server `BlenderConnection.send()` closes its socket per command so
  multiple clients can interleave (Claude Desktop + Claude Code, etc.).
- Addon `listen(1)` -> `listen(8)` so concurrent connects queue at the
  OS level instead of being refused.
- Commands still serialize on Blender's main thread via bpy.app.timers.
- New tests: socket released after send, two clients interleave.