Implement private NFT claims

[tindzk]

A public NFT claim proves a Merkle leaf built from the claimant, so the mint is tied to the player
who earned it. Add a path where a claim proves ring VRF membership of the claimants that registered
for a game: the proof shows that some registrant earned the credit, but not which one.

A game opts in at scheduling time and is then private only. Each claimant registers a one-time ring
VRF key on the People chain for a credit price, and the offchain worker builds one ring over the
registered keys once registration closes. The ring goes to the claims chain over XCM, where a
claimant submits one claim_private per slot the game grants. Each proof runs under a context
derived from (game_index, slot), and the alias it yields is the nullifier that spends that slot.

claim_private carries no signer and pays no fee: two claims from one fee payer are two claims by
one member. The proof authorizes the call and MaxPrivateClaimsPerBlock caps the verifications one
block runs. A ring's claims run in one window, which opens PrivateClaimDelay after the ring
arrives and closes PrivateClaimWindow later, and a member who misses it mints nothing.

A game that too few claimants registered for, or whose ring fails to build, is abandoned. Its
credits mint over the public path instead.