Firefox kills "idle" SharedWorkers under memory pressure with no native
signal to the client. The page's port silently becomes a no-op and
events stop flowing — observable symptom: mara's "dashboard never
refreshes; F5 fixes it" (because F5 creates a fresh page that creates
a fresh worker).
The worker now pings every connected port every 30s. The client tracks
last-activity-from-worker on every message arrival (incl. pings, since
those carry no URL — bumped before the URL filter in the route handler).
A visibility-gated watchdog polls every 15s; if the page is visible AND
has active subs AND hasn't heard from the worker in >90s, it presumes
the worker dead, logs a console warning, and re-subscribes on a fresh
port. Three pings missed before we act, so a normal tab-throttle blip
doesn't false-positive.
The fresh-port re-subscribe re-uses the bfcache-restore code path
(same shape: drop stale listeners, getSharedPort → new SharedWorker,
re-attach each route + repost subscribe). Recovery is per-tab — when
one tab's watchdog fires and brings up a new worker, other tabs that
share the named worker pick it up on their own watchdog cycle.
Falls back gracefully on environments without SharedWorker (the
existing direct-EventSource path is untouched) and is invisible on the
healthy path — pings are 30s apart, no UI surface.
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