Create Fly: Steam 'n' Rails Continued

Fixes

• Fix train coupling leaving the rear carriage without wheels, unable to move or be controlled until reconnecting. Client train packets now use Create Fly's client railway manager; previously they looked up the rear train in the server manager after the server had already merged and removed it.
• Fix client-side uncoupling updates potentially modifying an already-split integrated-server train a second time.
• Default registerMissingTracks to true on fresh installations. Early block registration and generated common.json now use the same default, so the first startup and subsequent restarts register the same integration tracks.