listens.from was a manifest constant — one value for every node a module runs
on. Now a per-node setting overrides it: {"expose": {"5432": "anywhere"}} makes
postgres public on the machine it is set for while it stays from:mesh elsewhere,
and the firewall (ADR 0050) is computed from the effective source. Exposure()
validates it — a port the module does not listen on, or a source that is not
mesh/anywhere/machine, is refused rather than reaching nothing; UnusedSettings
knows 'expose' is a real destination. Tested: default mesh, setting opens it to
anywhere, bad settings refused.