Test build 17: 2.4-fork3 — claimed DX7 banks start empty

Same as test build 16 (eight DX7 voice banks by name, the jump by kind, KEEP), plus: a bank newly claimed from USR4 is erased once at the first start of the build that claims it, so voices an earlier firmware left in that flash no longer appear as a full bank. Install it before loading anything into banks 5–8. Not a SLOOP release.