Do not create text range for labels of irrelevant nodes

### Description

Generating labels can be expensive so it should be deferred until it's able to be displayed. For example...

```xml

```

In this example which is simplified from a real world form, the `/data/note` is not relevant on load, and yet the `/data/concatenated` calculation is executed. This filters a large external secondary instance, and performs the join and concat, but it will never be displayed because it's only relevant when `/data/someid` is not empty which will completely change the filter results.

This evaluation is triggered in two places in the PrimaryInstance. Once when `attached` is set to true which reactively fires `createTextRange`, and once when `setChildren` is called (but I haven't tracked down the reactive graph to work this one out).

Found while working on https://github.com/getodk/web-forms/issues/844

Start by tracing PrimaryInstance's attached and setChildren paths, focusing on the reactive calls to createTextRange. Compare the behavior with the irrelevant /data/note example and issue 844. Done means labels for irrelevant nodes do not trigger text-range creation until they can be displayed.

Written by the indexing model from the issue text.