docs(gengo): explain why _v needs block scope in array compound-assign

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

compiler/gengo/emit_stmt.go

@@ -461,7 +461,14 @@ 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.
-		// Scope _v so multiple compound assigns in the same function don't redeclare.
+		//
+		// Wrap in { ... } so the _v binding is scoped to this snippet.
+		// Without the block, a function with two or more arr[i] op= rhs
+		// expressions emits `_v := t.Pop2()` twice in the same Go scope,
+		// which the Go compiler rejects with "no new variables on left
+		// side of :=". The other gengo sites that introduce _v (see
+		// gengo.go:1335, 1341 and emit_stmt.go:135) use the same block
+		// pattern; this branch was the lone exception.
 		g.writeln("{ _v := t.Pop2()")
 		g.emitExpr(idx.X)
 		g.emitExpr(idx.Index)