Rename `ZOwnablePK` to `ShieldedOwnable` and decouple owner identity from Zswap

[0xisk]

The ZOwnablePK name implies the identity model depends on the Zswap coin
public key, but the privacy guarantee comes from the secret nonce. The Zswap
key is incidental.

Proposed:

• Rename module to ShieldedOwnable (update refs, tests, docs).
• Refactor owner-id logic to drop ZswapCoinPublicKey.
• Breaking change: note migration implications.