Review round 1. A bind failure after subscribing was logged once as
"waiting for the feed bus". session() now ends as Unreachable, BindFailed or
BusLost, each logged as what it is (once per streak), and
fly_edge_bind_failures_total counts the second; a new parity test holds the
port, sees the edge fail to bind without claiming to serve, frees it and gets
served.
feed.bus_dir (FLY_BUS_DIR) must be absolute and non-empty, checked in either
mode, since flysim and the edge each resolve it and a relative path would
let them disagree.
The sizing worst case was 7 stuck subscribers; that seat does not exist,
since edge.sock admits one client. It is that client with all 4
subscriptions it may open: 4*3+1+2 = 15 snapshots, about 1.8 MB. feedbus's
comment and unit test say so, and stall.rs's hoarding scenario now takes all
four subscriptions, checks a fifth and a second connection are refused, and
bounds the store at 15 snapshots.
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