fix(gengo): scope _v in array compound-assign to avoid Go redeclaration

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>

compiler/gengo/emit_stmt.go

@@ -461,11 +461,12 @@ func (g *Generator) emitAssign(a *ast.AssignExpr, locals localMap) {
 		g.emitExpr(a.Right)
 		g.emitBinaryOp(a.Op)
 		// Stack now: [folded value]. Re-push X/index to set.
-		g.writeln("_v := t.Pop2()")
+		// Scope _v so multiple compound assigns in the same function don't redeclare.
+		g.writeln("{ _v := t.Pop2()")
 		g.emitExpr(idx.X)
 		g.emitExpr(idx.Index)
 		g.writeln("t.PushValue(_v)")
-		g.writeln("t.ArrayPop()")
+		g.writeln("t.ArrayPop() }")
 		return
 	}