feed.via (FLY_FEED_VIA, default direct) and feed.bus_dir (FLY_BUS_DIR,
default /run/fly/bus). In bus mode flysim does not bind the feed port: it
starts a router on its own two-thread runtime with a closed policy (flysim
may publish fly.feed.snapshots, fly-edge may only subscribe to it), listens
for the edge on <bus_dir>/edge.sock, and a publisher task copies each snapshot
out of the watch slot onto the latest-retained topic: frame, audio and spikes
as sealed artifacts, the header as the envelope payload, or as a header
artifact past 48 KiB. The sim thread still only writes its watch slot, so a
slow bus costs snapshots on the bus and never a frame of the loop.
feedbus holds both halves of the encoding, publish and receive, and the
limits sized for the real 122,367-byte snapshot. Two counters are new:
fly_bus_published_total and fly_bus_publish_failures_total.
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