Should legacy mode be explained in the tutorial?

[brunnerh]

As long as legacy mode exists, there is a reasonable chance that it will cause problems.
I have seen a lot of confusion leading up the release of Svelte 5 due to the automatic mode switches.

If new projects are initialized with a global flag for runes mode, I think it would be fine to not explain it, but last time I checked, this would cause issues with component library dependencies that are legacy since the flag affects those as well, unless dynamic compiler options are used which exclude node_modules.

I think the lack of fine-grained reactivity in legacy is not that much of an issue, but people might work on imported, unreactive data without any runes and assignments trigger reactivity via legacy mode, so everything looks correct but when they later introduce a rune it mysteriously breaks.

Relates to:

• default to runes mode in tutorial #1273