Add randomized raft simulation seeds

[kacy]

Summary

Add a deterministic randomized raft simulation harness with fixed seeds.

What Changed

• add a seeded 3-node random event runner in src/test_cluster_sim.zig
• cover transport-fault toggles, restarts, healing, and leader proposals
• assert raft progress and convergence invariants after randomized event sequences
• print the seed trace on failure to keep runs reproducible

Validation

• YOQ_SKIP_SLOW_TESTS=1 zig build test-sim -Doptimize=ReleaseSafe
• YOQ_SKIP_SLOW_TESTS=1 zig build test-contract -Doptimize=ReleaseSafe
• YOQ_SKIP_SLOW_TESTS=1 zig build test-integration -Doptimize=ReleaseSafe
• YOQ_SKIP_SLOW_TESTS=1 zig build test -Doptimize=ReleaseSafe still needs follow-up because the suite can exit nonzero from logged-error accounting even when assertions pass
• YOQ_SKIP_SLOW_TESTS=1 zig build test-gpu -Doptimize=ReleaseSafe still needs follow-up for the same logged-error exit behavior