BREAKING CHANGE: `pda_accounts` field in NSSAUserData renamed to `shared_accounts`. `PrivacyPreservingAccount` enum has a new `PrivateShared` variant, exhaustive matches must handle it.
- Add SealingPublicKey/SealingSecretKey type aliases for seal_for/unseal
- Generalize PrivateGroupPda to PrivatePda with pre-resolved keys
- Rename group_pda_spender to private_pda_spender
- Rename group_pda_accounts to pda_accounts with serde alias
- Remove unused storage_mut()
- Remove stale group_pda_router.bin artifact