every level load left another handler attached to a freed HUD.
SpeedRunManager outlives the scene, so it kept calling them and each one
wrote to a disposed Label - thousands of ObjectDisposedException per
frame, growing with each load.
Latent all along: StartTimer was only ever called from StartNewGame, so
after a Continue the timer never ticked and the dead handlers never ran.
Calling StartTimer on Continue (needed for splits and best times) made
it visible.
Also stop the timer when leaving to the menu - StopTimer existed and had
no callers, so the run kept ticking in the main menu.
