* [debug] fix port collision in TestUpload
TestUpload hardcoded :8080, so it failed deterministically when anything
was already on that port and collided across concurrent test runs.
Bind a :0 listener in the test to get a kernel-assigned free port, and
add Server.Serve so tests can hand the listener in without reaching
into unexported state.
* [debug] drop test-only Server.Serve, use SERVER_ADDRESS env
The previous commit added a Server.Serve method on the upload-server,
used only by TestUpload. That left production with an unused function.
Reserve an ephemeral loopback port in the test, release it, and pass
the address through SERVER_ADDRESS (which the server already reads).
A small wait helper ensures the server is accepting connections before
the upload runs, so the close/rebind gap does not cause a false failure.
