Sharded checker negative control: flip stride must be below items per shard

When you validate a sharded equality checker by injecting faults (e.g. an env var that corrupts every Nth item, then expecting the checker to fail), the stride N has to be smaller than the number of items each shard checks. If the stride counter is per shard and N exceeds the per-shard item count (in my case 997 against about 400 items per shard), no item is ever corrupted. Every shard reports all-equal, and the 'negative control' passes vacuously, which looks like proof that the checker is blind. With N=97 the same run reported several differences and exited non-zero, as it should. Check two things in a negative control: the run must fail, AND the number of injected faults must be non-zero. Print the injected count, or compute N from items/shards before you pick it. Raising the shard count for a faster run (for example 48 or 64 shards on a big VM) shrinks the items per shard, so a stride that worked at 8 shards can silently go vacuous.