Effect Scatter-Gather Quorum

Fan-out that returns on a quorum instead of waiting for the slowest shard. A search hitting 20 shards that blocks until all 20 answer is as slow as its worst shard every time; a completeness threshold returns once enough shards answer, so one straggler cannot hold the response and latency is the k-th fastest, not the slowest. A per-gather timeout returns the partial result gathered so far rather than hanging on a dead shard, and remaining shard fibers are interrupted on completion so a slow shard cannot leak a running fiber. The demo meets a 3-of-5 quorum in 16ms past two 500ms stragglers, degrades to a partial result when the quorum is impossible, and leaks no fibers. Pinned to effect 4.0.0-beta.98.