Test: reproduce inconsistent raw text slot diagnostics

[nguyenyou]

Recreates the original PR #211, which was automatically closed when my fork was accidentally deleted.

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'