reject named-field enums in SoAble derive to avoid unsound layout

[paradoxicalguy]

enums with named fields are currently laid out positionally, which can
pair unrelated fields across variants and cause unsound behavior.

this PR explicitly rejects named-field enums in the SoAble derive macro,
emitting a clear compile-time error explaining that a field-name–based layout is required.

this avoids unsoundness while leaving room for a correct implementation in the future.

fixes #43 by rejecting the problematic case.

[aapoalas]

I'd rather see the named field cases fixed than to remove the support; right now they work and are generally fine in normal cases when the user has laid them out in systematic order.

Also thank you for the contribution and hello! :)

[paradoxicalguy]

hello :) and thank you for the clarification

i'll take a look at implementing a correct field-name-based mapping for enums with named fields (so that fields are paired by name rather than position) and update the PR accordingly.

[paradoxicalguy]

i’ve updated the derive to implement a field-name–based SoA layout for enums with named fields, pairing fields by name rather than by position.
all tests passed; would appreciate another look!