saveRegistryConnection only rewrote the registry file: editing a
connection's URL/token/host left pooled backend descriptors under
'conn:<id>::*' and open renderer sockets pointing at the OLD endpoint —
the UI showed the new target while traffic kept flowing to the old one
until idle-reap.
When a save MATERIALLY changes an existing connection (endpoint / auth /
ssh routing fields, via the new connectionDialFieldsChanged helper),
main now stops that connection's pooled backends and tunnels
(stopRegistryConnectionBackends, same teardown as removal) and
broadcasts 'hermes:connections:changed' with reason 'updated' so
renderers dispose and re-dial their secondaries at the new target.
Label-only renames do not recycle.
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