Figure out the exact semantics of `apply` in edge cases

[SolalPirelli]

e.g., parameterless apply, apply of a custom function-like type, ...

see #25463 (comment)

a Slack discussion yielded basically "we shouldn't allow the automatic use of parameterless def apply: ... = ...

[som-snytt]

This came up not long ago; I commented on my PR that apply was not intended to be a parenless property. I don't yet recall what that was about, but maybe an error message? or something else about paren insertion?