A pure read names the tests failing on three or more open pulls: a clash between pulls, not separate bugs #6551

Part of #5930 and #5994: nine pulls once went red on one size test, and each was handed back alone.

Done:

• [ ] New src/flow/shared_red.ts exports sharedRed(rows), taking { pull, failingTests: string[] }[] and returning { test, pulls: number[] }[] for each test failing on 3 or more pulls, most pulls first.
• [ ] sharedRedSays(rows): "<test> fails on 9 pulls: one clash, re-check together after the fix".
• [ ] Tests: below the threshold, at it, ordering, a pull counted once per test.

Files: src/flow/shared_red.ts, test/flow-shared-red.test.ts.

Before pushing: bun x tsc --noEmit and the new test.