V21: Add fence (monotonic ownership generation) to cyclium_work_claims.

The fence increments on every acquire/steal/reclaim of a claim. A running episode holds the fence it acquired; before delivering outputs it re-checks that the live claim still carries that fence (via Cyclium.WorkClaims.gate_owns?/3). This catches a lease that was stolen — and possibly re-acquired by the same node (A → B → A) — while a stalled node was mid-run, closing the residual double-delivery window left open by the owner-only ownership check.

Existing rows get fence 0; new claims start at 1.