The chat.js fetch wrapper recorded a request-not-found from /api/requests/<id>/workspace as a missing request and then served a synthetic 404 to the request panel's rereads, failing 11 request-browser tests. Only the request's own read records a miss now. Adds a regression test that fails on 9d3eb9e11, and bounds the r3 held-404 test's check on its refresh reads, which raced Files' cached repaint under load.
A mosaic-request-state for an id now advances that id's own generation on every thread retaining it, so a 404 still in flight from before the observation cannot record the miss afterwards. Generations are pruned with the misses to retained rows. Adds a held-404 regression that fails on 16384bb12.
jason.woltje