In order to address feedback that the "Stable ID" was not easy enough
to use, this commit removes the "Stable ID" output from the distribution node,
and the input from the instance on points node. Instead, the nodes write
or read a builtin named attribute, id.

The downside is that more behavior is invisible, which is less expected
now that most attributes are passed around with node links. This behavior
will have to be explained in the manual.

The random value node's "ID" input that had an implicit index input
is converted to a special implicit input that uses the stable ID if possible,
but otherwise defaults to the index. There is no way to tell in the UI which
it uses, except by knowing that rule and checking in the spreadsheet for
the id attribute.

Because it isn't always possible to create stable randomness, this attribute
does not always exist. And it will be possible to remove it when we have
the attribute remove node back, to improve performance.

Addresses T92005

If it isn't possible to tell based on the patch description, I'm conflicted
about whether this change is actually better. But we did decide on it earlier,
and it means new users will have less to think about anyway.