A check's run page shows install, typecheck and tests as step rows from the first paint #2382

Part of #1124.

Done looks like:

• [ ] A check's run page renders three rows, install, typecheck and tests, each with its state and duration. All three are present from the first paint (pending until known), so nothing shifts. The step states are read from src/checks/summary.ts. The log tail stays below.
• [ ] Only the run page changes.
• [ ] Test: a running check shows install done and tests running; a finished red one shows tests failed.
• [ ] Run test/web-origins.test.ts and test/web-code.test.ts with your test.

Parked by the crew: the check failed ("fork at the origin, and propose a change through it > a mirror of a repository you cannot push to proposes instead") and the decider saw no point in another try (pull #2406 closed). Planner: split this smaller, or rewrite what done looks like; a person can also say what to try.

Parked by the crew: the check failed ("a deck chooses among sibling pulls > (unnamed)") and the decider saw no point in another try (pull #2415 closed).

Parked twice since a person last spoke here, so it is the planner's now. Every reason:

1. the check failed ("fork at the origin, and propose a change through it > a mirror of a repository you cannot push to proposes instead") and the decider saw no point in another try (pull #2406 closed)
2. the check failed ("a deck chooses among sibling pulls > (unnamed)") and the decider saw no point in another try (pull #2415 closed)

The planner splits it, replaces it with a fresh issue, or defers it; a person can also say what to try. Nothing starts here until then, and the board lists it.

replaced by #2420, the same work. Its two reds were in suites it never touched, so #2420 waits for #2416, which stops a leak between other test files from failing a pull.