Test: reproduce inconsistent raw text slot diagnostics

[nguyenyou]

Adds a reproducer for inconsistent diagnostics when a text <-- inserter is used inside a named Slot.

Observed behavior:

• Initial placement renders the text in the default slot without reporting an error.
• Moving the same inserter into another named slot reports a raw-text-in-named-slot error.
• The text remains in the default slot and stays reactive in both cases.

This test documents the inconsistency without prescribing whether the initial placement or the move should change.

Run:

sbt 'testOnly *TextInserterSlotMoveSpec'

[raquo]

A text node can't belong to a slot, since we can only set a slot attribute on elements.

So we render the text node in the default slot regardless of where it's slotted and regardless of the errors we report.

But, reporting the error does seem desirable in this case – the user needs to know that they're trying to incorrectly put text into a slot that can't accept it.

[nguyenyou]

Thanks for clarifying; I updated the PR to document the inconsistency.

[raquo]

Ah, I see. I'll need to look into it later but OTOH putting text <-- inside slotted children <-- should warn the first time too. Not sure why it doesn't.

[nguyenyou]

The inconsistency seems to come from ChildTextInserter passing slotName = () initially, while moves use NestedGroup.moveToParent and eventually call TextNode.applySlot.