One test proves #477 end to end: a degraded forge says 'needs you: forge …' on the board, pages the phone once, and once more when it clears #2526

Replaces #477, a person's ask whose pieces all landed (#2323, #2442, #2443, the drain page in ac2dd46f, #2492). Each piece has its own test (test/guard-verdict.test.ts, test/board.test.ts, test/notify-drain.test.ts, test/notify-causes.test.ts), but nothing proves the owner's story in one place.

Done looks like:

• [ ] A new test/forge-degraded.test.ts drives one owned cause (for example errors) past five minutes with a fake clock and a fake notify endpoint, then asserts three things: the board page's forge strip says 'needs you: forge …', exactly one page was sent, and nothing more is sent on further ticks.
• [ ] When the cause clears and stays clear for five minutes, exactly one Recovered page is sent and the strip says all fine.
• [ ] Only the test is added. If it fails, fix the code it finds and name the fix in the pull.