Shards: a failing file is run once more alone, and one that passes alone does not fail the check #2445

Part 2 of #2416. It wires besideOthers (part 1) into scripts/test-shards.ts runShard.

Done looks like:

• [ ] After a sliced shard goes red, each of its failing files (at most 3) is run once as its own bun test with the same env and memory limit. The verdict comes from besideOthers, and its line is printed in the tail. The whole-gate path, which already runs one file per process, is unchanged.
• [ ] Tests in test/shards.test.ts with two fixture files under test/fixtures/, kept out of the gate: one sets a global that breaks the other only when they share a process. That shard comes out green and names the file. A fixture that fails alone stays red.
• [ ] Both pulls for #2416 went red on files they never touched (toolbar actions, quotas in the terminal). If the forge's check is red on such a file, name it in the pull and do not change it.