Only get errors on save

I'm sure this must have been discussed somewhere before, alas I couldn't find such an issue here: is it possible to only get error messages on save or at least with some delay? I find it hard to concentrate when every keystroke causes big flashes of red.

Start by reviewing the existing discussion and the Haskell Language Server's current diagnostic behavior while editing. Clarify whether diagnostics should appear only on save or after a delay, then identify the relevant implementation and tests; done requires an agreed behavior that avoids disruptive errors while preserving useful feedback.

Written by the indexing model from the issue text.