Let a parser report that a page states it lists no events

A count of zero has two causes and nothing downstream can tell them apart:
a source whose selectors all broke, and a game that is simply between
patches. Both parse to nothing, so `canParse` plus a row count cannot
separate them — and the pipeline currently has to assume the worst, which
is right for a redesign and wrong for a quiet week.

`statesNoEvents` is the seam for the one thing that can settle it: the
page's own words. Optional, because most pages say nothing either way, and
absent means the strict gate stands.

No parser implements it yet and nothing reads it — the contract widens here
so the parser and the refresh gate can land as their own changes.

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

src/ingest/adapters/index.ts

@@ -227,6 +227,9 @@ function toAdapter(spec: SourceSpec): Adapter {
     parserId: spec.parserId,
     minIntervalMs: spec.minIntervalMs ?? SIX_HOURS_MS,
     priority: spec.priority ?? 0,
+    statesNoEvents(html: string): boolean {
+      return parser.statesNoEvents?.(html) ?? false;
+    },
     parse(html: string, ctx: ParseContext): GachaEvent[] {
       // A site redesign should fail the run loudly rather than publish an empty
       // calendar, which would read as "no events" to a user.