Regression: keyed rule-view sharing, a write costs 11.9 s CPU and a 643 ms ack at 1M edges / 1,000 keys (was 0.18 s / 3.8 ms) #378

Found by the views benchmark (#308, PR #369) on the benchmark host: it reproduces the architecture review's baseline on the review's commit within noise, but current main regressed keyed rule-view sharing: at 1M edges with 1,000 keys, one write costs 11.9 s CPU and a 643 ms ack, against 0.18 s and 3.8 ms before. Suspects merged since: #355 (session throughput, round dedup), #303 (shared probe round fix), #282 (parameterised programs). Bisect on the benchmark host, then fix or revert the culprit; add this case to the perf gate so it can never regress silently again.