_start_channel built the default seen-event store path inline, and
Paths.base_dir resolves through realpath. Channel startup reaches that
wiring from POST /api/channels/{name}/restart, so every restart ran a
filesystem syscall on the Gateway event loop; the rest of the same
function already keeps its disk IO in a worker thread.
The existing wiring test stubs get_paths with a plain-string base_dir,
so it cannot see the syscall. The new anchor keeps the real Paths object
and isolates it with DEER_FLOW_HOME instead.
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