Handle ref pin patterns in borrowck

[P8L1]

This follows up on the review comment from #5 asking for borrowck coverage for ref pin patterns.

The new tests showed that ref pin mut / ref pin const patterns were not being treated equivalently to direct &pin mut / &pin const borrows. With only the new tests kept and the implementation files reverted, the new diagnostics were missing.

This change makes pinned ref patterns lower to BorrowKind::Pinned(..., PinBorrowKind::Persistent), while preserving the existing behavior for ordinary non-pinned ref / ref mut patterns.

Added coverage for:

• ref pin mut followed by a later &mut
• ref pin mut followed by a later move
• ref pin const followed by a later &mut
• ref pin const followed by a later move
• pinned projection cases
• pinned parent / field cases
• Unpin vs !Unpin behavior

[P8L1]

@frank-king This should address the remaining "ref pin" / borrowck coverage you requested in #5.

Since rust-lang#153693 is still draft and the "Test pin pattern matching" task is still open there, would you prefer to merge this follow-up into "feature/pin-borrowck" before you do the final self-review / mark the upstream PR ready for review?

No rush; just checking so I do not add noise in the upstream PR if you want this integrated first.