fixes race condition where $IPFS_PATH/api and $IPFS_PATH/gateway files
were written before the HTTP servers were ready to accept connections.
this caused issues for tools like systemd path units that immediately
try to connect when these files appear.
changes:
- add corehttp.ServeWithReady() that signals when server is ready
- wait for ready signal before writing address files
- use sync.WaitGroup.Go() (Go 1.25) for cleaner goroutine management
- add TestAddressFileReady to verify both api and gateway files
