emitAssign for arr[i] op= rhs emitted bare _v := t.Pop2(), so functions
with multiple compound assigns to array elements failed to build with
"no new variables on left side of :=". Wrap the snippet in a block so
each occurrence has its own _v scope (same pattern as gengo.go:1335).
Repro: any PRG with two or more arr[i] += x or arr[i][j] += x in the
same function (e.g. labdb session-stats.prg with 6-channel peak
accumulators).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>