fix(MPC deadlock): give each RCOT consumer its own instance

[AdamDawidKrol]

MPC preprocess (crates/tlsn/src/deps/{prover,verifier}.rs) builds one SharedRCOT and hands clones to the concurrent try_join3 branches (key-exchange / record-layer / vm). A SharedRCOT only performs its single batched flush once all live clones reach the flush barrier — but the branches don't all flush in the same round, so a clone whose flush() is parked behind a sibling branch never arrives. The barrier then waits forever (alive=N, arrived=1) → an intermittent deadlock during preprocessing.

Fix:
Give each RCOT consumer its own SharedRCOT instance instead of clones of a shared one, so every barrier rendezvous is independent and can't wait on a clone parked in another branch. All senders keep using the same global delta.

[sinui0]

This introduces a subtle security flaw: the kos protocol gets instantiated with the same delta correlation and the receiver has the opportunity to re-use the same base OT setup. Our kos implementation does not do domain separation by instance, so I think this gives the receiver the opportunity to outright learn (fatal) or at least sample delta.

Two options:

1. Add instance domain separation to the kos implementation
2. Use independent instances (run base OT twice)

[AdamDawidKrol]

@sinui0 Thank you for the comment! I'll answer in a few days.

[AdamDawidKrol]

@sinui0 I reproduced the attack you described as a test in mpz: AdamDawidKrol/mpz#1

But I've been trying to see how a malicious party actually accesses the data it needs, and I can't. The recovery reads the honest sender's COT keys (keys_A ^ keys_B = delta). In the tlsn setup the malicious party is the receiver, and its own output is the choice-independent value (delta-free) — the sender folds the receiver's choice into its keys, so flipping choices across two instances just returns the same masked value, not the complementary key. So I don't see a path where prover or notary observes the delta-bearing keys.

Am I missing something?

[AdamDawidKrol]

Domain separation pushed (privacy-ethereum/mpz#446 + commit here)