Harden the dev route-miss reconciliation
Address adversarial review findings on the previous commit:
- A route deleted between watcher updates could match a stale dev matcher
and fail the request with 500 instead of rendering the 404 page. Treat a
PageNotFoundError from ensure as no match.
- Concurrent matcher reloads (now reachable from the request path) ran
unserialized; coalesce them into one running and one queued scan, and
don't let a failed reload reject every subsequent request through
waitTillReady.
- The router 404 probe missed rewritten requests when
useFileSystemPublicRoutes is disabled (gate on didRewrite), ran for
/_next/* asset paths, and missed routes requested with a trailing slash.
- ensurePage now re-checks the entrypoints once when its wait deadline
fires, so an update landing at the deadline isn't dropped.
