Fix race condition in RenderSocket

[715d]

Socket.currentRender was read and written from multiple goroutines (WebSocket reader, broadcast handler, upload POST handler) without synchronization. The render, diff, send, and update steps are also not atomic — a concurrent event can interleave between diffing and updating the stored render tree, producing corrupt patches.

The same applies to Socket.uploads and Socket.uploadConfigs, which are accessed from both the HTTP POST upload handler and WebSocket event handlers concurrently.

I added a new test, socket_race_test.go, which triggers the race detector.

I took the simple approach for the fix, which was to introduce a mutex to protect both the render and upload state.

I added Socket.Render(ctx) which is the replacement for RenderSocket, using the mutex to synchronize the render->diff->patch cycle.

A more controversial change might be that I changed RenderSocket, UpdateRender, and LatestRender to be un-exported, so that an outside caller can't do the wrong thing accidentally.

I also changed UploadConfigs and Uploads to return copies so the caller can't mess with the slice that might be concurrently changed by AssignUpload/ClearUpload/etc