Multi-root servers (pyright) are keyed by server_id; a file whose resolved
root is new for a running client is attached with
workspace/didChangeWorkspaceFolders instead of spawning another server.
Single-root servers keep the (server_id, workspace_root) key and behavior.
A profiled fan-out across ~30 worktrees ran 30-60 pyright processes
(~8.7 GB); the same fan-out now runs one.