Per-bar decode-ahead: mid-gen instrument swaps no longer bake silent bars

The device audit caught the remaining "random mute" exactly: a sampler→sampler instrument swap inside a gen (holdUntil churn — "new hands on the pads") keeps the topology sig, so there is no reopen, and only reopen decoded buffers

• The ring path never had this because it decodes what each bar needs as it's fed. Ported: before feeding a bar, the wav pump kicks any missing zone/found PCM through the decode gate and HOLDS that bar until it…

Verified: live-resilience, audit-gate, wavout (all routes), live, segment-parity byte-equal, engine.

▶ Play it: Ballroom Leviathan — a horn-and-piano genre that plays almost entirely sampled instruments https://stellate.app/?genre=bigband&seed=5482&path=814.14134,833.14167,795.14167&xdur=13.122

985c247 https://github.com/aboard-io/stellate/commit/985c247a89566f98dbb5738f73f8687b5787f076