DealSession

Drives a cryptographically fair card deal over a Seam, as an op-based CRDT.

Each card is a CardState; players sequentially encrypt the deck (SRA layers compose because the cipher is commutative), then non-quorum players strip their layer so only the visibility quorum can read each card. State converges via the CardOp stream broadcast over the seam.

Single-writer assumption. The shuffle and strip helpers read the base ciphertext from state outside the _state.update block, run crypto, then commit. If the incoming-op collector races to apply a remote op to the same card in that window, the commit can overwrite converged state with a stale-base ciphertext (canApply checks GSet membership, not ciphertext provenance). This is safe under the current model where each session has exactly one local writer; concurrent local shuffle/strip while remote ops mutate the same card is a production hazard that needs an "await my turn" gate (deferred).