Support or reject generic persisted validator impls

[eboody]

Summary

Generic persisted validator impls do not appear to round-trip cleanly through #[validators] expansion.

The macro rebuilds emitted impl blocks from self_ty instead of preserving the original impl generics and where-clause. That looks fragile or broken for shapes like impl<T> DbRow<T>.

This also appears to predate the non-authoritative scanning refactor, but it should be covered explicitly.

Current shape

The validators path keeps item_impl.self_ty and emits new impl blocks from that type. The original item_impl.generics are not preserved on the emitted outer impls.

Relevant code:

• statum-macros/src/validators.rs
• statum-macros/src/validators/emission.rs

Expected behavior

If generic persisted types are meant to be supported, the macro should preserve:

• impl generic params
• where clauses
• direct compatibility with generic persisted types used by #[validators]

If they are not meant to be supported, the macro should reject them explicitly with a clear diagnostic.

Suggested acceptance

• add a trybuild case for a generic persisted validators impl
• either support it fully or fail closed with a targeted error
• document the supported shape in validators docs