The avatar cache files written for Communication Notifications cannot be
deleted right after the handler returns (the system reads them lazily, e.g.
the Apple Watch fetching the image), and the NSE gets no dismissal callback.
Add best-effort, age-based cleanup that drops files older than 24 hours each
time a new avatar is written. Notifications are ephemeral, so any device that
needed an old file is long done. The cleanup tolerates concurrent NSE
instances removing the same file.
Claude
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