Use real nullifier count in Unlink event model

[Th0rgal]

Summary

• count nonzero nullifiers before the transfer/withdraw event surrogate
• use that filtered count for the _realNullifiers(...) event payload approximation

Tests

• lake build Benchmark.Cases.UnlinkXyz.Pool.Compile
• git diff --check

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Note

Low Risk
Low risk: only changes event payload data to use a computed non-zero nullifier count; no state-transition logic or verification paths are modified.

Overview
Updates the UnlinkXyz pool Verity contract to emit a real nullifier count (computed via countNonZero over txn.nullifierHashes) in the Transferred, Withdrawn, and EmergencyWithdrawn event surrogates, replacing the previous arrayLength-based approximation.

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