setPendingKey

Marks or clears the pending flag for an arbitrary hash key, independent of a specific Tile instance. A multi-interval batch request marks only one "representative" tile pending, and that flag clears the moment the HTTP response arrives — well before the other intervals in the batch have been decoded, edge-blended, and queued to bind. Until this per-interval flag is set too, isInFlight sees those other intervals as safe to drop, so a concurrent clearOutsideBounds or evictTimesNotIn sweep can strand one before it is ever bound. placeholder is never read back — only key presence matters to pending/in-flight checks.