feat: CAT minting + nullifier v2 (PR5)

[almogdepaz]

Summary

• CoinMode::Mint implemented across all backends (mock, risc0, sp1) — was panic/error
• compute_genesis_nullifier: one-time-use genesis coin enforcement via SHA256("clvm_zk_genesis_v1.0" || serial || tail_hash) — prevents double-minting
• compute_nullifier_v2 (ZK-01 fix): SHA256("clvm_zk_nullifier_v2.0" || tail_hash || serial || program || amount) — tail_hash binding prevents cross-asset nullifier collision attacks. v1 deprecated.
• mint_cat simulator method + mint_command CLI
• CoinInfo.tail_source: stored at mint, auto-retrieved at spend — CAT coins now spend correctly via spend_coins
• 6 new tests covering: unlimited TAIL, genesis nullifier, double-mint prevention, nil TAIL rejection, hash mismatch, mint-then-spend

Files changed (13)

• clvm_zk_core/src/lib.rs — compute_genesis_nullifier, compute_nullifier_v2, v1 deprecated
• backends/mock/src/backend.rs — full Mint arm, enforce_ring_balance guard, v2 nullifier
• backends/risc0/guest/src/main.rs — Mint arm, v2 nullifier
• backends/sp1/program/src/main.rs — mirror risc0
• backends/risc0/src/lib.rs, backends/sp1/src/lib.rs — removed host-side Mint guards
• backends/risc0/guest_settlement/src/main.rs, backends/sp1/program_settlement/src/main.rs — v2 nullifier
• src/lib.rs — ClvmZkProver::prove_with_input
• src/simulator.rs — mint_cat, CoinInfo.tail_source, UTXO key fix, CAT spend tail_source lookup
• src/cli.rs — SimAction::Mint, mint_command
• tests/test_cat_minting.rs — 6 tests

Test plan

• cargo test --no-default-features --features mock,testing --test test_cat_minting — 6/6 pass
• cargo test --no-default-features --features mock,testing — full suite green, no regressions