errgroup: rewrite worker scheduling with sync.Cond to fix data race and delayed task dispatch

[noble-gase]

Replace the channel-based worker scheduling with a sync.Cond based implementation.

Fixes

• Data race: Go() mutated cache/remain without locking; all shared state is now mutex-protected.
• Delayed dispatch: queued tasks were only flushed inside Wait(); they are now consumed immediately by idle workers via cond.Signal().
• Lost cancellation cause: return context.Cause(ctx) instead of ctx.Err().

Improvements

• Spawn workers on demand only when backlog exceeds idle workers.
• Calling Go() after Wait() now panics with a clear message.
• Nil out dequeued slice elements to avoid memory retention.

Testing

go test -race ./errgroup/