Tell a broken source apart from a stale one

CI failed on Infinity Nikki yielding no events, and was wrong to. Its
snapshot parses to six events; every one of them had ended by the morning
the build ran. The parser is fine — `--now 2026-08-14` gives six, today
gives none — and the page simply has nothing current left on it.

eventCount is counted after expired events are dropped, so "this parser has
stopped reading a redesigned page" and "this page's events have all
finished" arrived as the same zero. Only the first means our code is wrong,
and only the first should redden a build. So the feed now records what each
document yields parsed as of its own capture date, before expiry, and the
check fails on that instead.

Nikki cannot refresh itself out of this, either: docs/SOURCES.md records
that Fandom refuses the Actions runner. A lane with an empty calendar is a
real problem, but it is a refresh problem, so it is reported on the build
log and left visible rather than thrown.

parsedCount is nullable and defaulted, never required: the client validates
the whole feed with safeParse and the service worker serves the last feed it
downloaded, so a required field would have made every cached feed fail
validation and taken the offline promise with it. Null also covers a source
whose bytes were never confirmed live — there is no date to parse "as of",
and a build is not failed on missing information.

The rule moved to shared/feed.ts. A test did pin the old one, by grepping
the workflow for the string — which proved the check existed, never that it
was right.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>

README.md

@@ -341,10 +341,16 @@ Until then the `pages` job is skipped and the pipeline stays green. Pages is una
 repositories on the free plan. Both steps are done here, and the deploy lands at
 <https://stereotypicalcat.github.io/gacha-event-tracker/>.
 
-The feed job fails if the event count collapses. A source that quietly stops yielding events is the
-failure mode a parser-only pipeline is most prone to, and nothing else would surface it. Tests run
-offline against checked-in fixtures, so a red pipeline always means the code changed rather than a
-wiki being down.
+The feed job fails if the event count collapses, or if any single source parses to nothing — nine
+healthy sources hide a tenth that has gone quiet, and the total stays comfortably over the floor
+while one game shows an empty calendar. That is the failure mode a parser-only pipeline is most
+prone to, and nothing else would surface it.
+
+It distinguishes that from a source whose events have all simply *ended*, which is a stale page
+rather than a broken parser and is reported instead of thrown. The two used to be the same zero,
+because the count was taken after expired events were dropped — so Infinity Nikki reddened the
+build the morning its last event finished. Tests run offline against checked-in fixtures, so a red
+pipeline always means the code changed rather than a wiki being down.
 
 ### Refreshing the data