Per-track loudness normalisation

The platform's loudness enhancer is constructed against an audio session id and stays attached to it for its whole life. That is fine for a single long-lived player. It is a trap the moment a new player is created per track: each new player gets its own session, and the effect you built for the previous one is still perfectly alive, still enabled, and no longer connected to anything you can hear.

The working shape is: on every track change, tear the old one down, build a new one against the current session, then push the gain the track's metadata asks for.