resolveAgentAvatar cached null permanently (window lifetime), so a bot
whose avatar reached the asset store moments after its first notice
rendered — freshly created bots, art backfills in flight — kept the 🤖
glyph until an app restart even though profiles.get_asset had the pfp
(user report: brand-new bots' notices never picked up their faces).
Hits stay cached for the window; misses re-probe after 30s. Same
dedupe/inflight behavior otherwise.
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