Fix seq[var T] payload layout; fixes #26058

[subotac]

Fixes #26058.

Treat the var element modifier as an access property when generating sequence payload types, keeping seq[var T] ABI-compatible with seq[T].

Add a regression test covering variant-object stride and in-place mutation.

[Araq]

keeping seq[var T] ABI-compatible with seq[T].

What? No, that's not good enough.

[subotac]

You're right — var T is still a reference element here, so stripping it only from the payload makes codegen inconsistent (Item** vs Item*). I'll drop this approach rather than patch it further in the backend.