The mesh may only move a port it actually publishes

The lab caught this: a module declaring a port and running no container
had its rule set opened on 20000 while its service sat on 9101. The
firewall reported success and blocked the thing it was told to admit,
which is the precise failure the filtering comment warns about, arrived
at from the other side.

Assignment was applied to every declared port. But a container's mapping
is the thing that translates, and where there is none the software binds
what it binds — the mesh choosing a number does not move the service, it
only makes the mesh wrong about where it is.

The declaration side already knew this: publishedOn rewrites container
ports and nothing else. Filtering did not, so the two disagreed about
the same fact. MachineSide is now the one derivation both follow.

It also fixes a second case nobody had hit yet: a mapping the manifest
wrote itself, like the mail system's 7080:80. That is passed through
untouched when composing, so assigning it a machine port would have
opened a rule on a port the container does not publish. Either side of
such a mapping now names it, and the host side is the answer — a module
may read `listens` as what its software binds or as what the machine
exposes, and both readings want the same number.

Recorded either way, assigned or not: the map means where this module's
port is on this machine, and every reader needs that answer regardless
of who chose it.

Tests bite — making it always assignable reproduces the lab failure.

cmd/mesh-control/plan.go

@@ -266,16 +266,26 @@ func declarationWith(ctx context.Context, open *stores, node string,
 	ports := map[string]map[int]int{}
 	for _, m := range plan.Modules {
 		for _, l := range m.Listens {
-			at, err := inv.PortFor(ctx, node, m.Module, l.Port, l.Fixed)
-			if err != nil {
-				return nil, fmt.Errorf(
-					"%s needs %d reachable on %s and it could not be assigned: %w",
-					m.Module, l.Port, node, err)
+			// **Only a port the module actually publishes is the mesh's to move.** A container's
+			// mapping is the thing that translates; without one the software binds what it binds,
+			// and an assignment would not move the service — it would open the wrong number in the
+			// rule set and leave the real one shut. Recorded either way, because this map means
+			// *where this module's port is on this machine* and every reader of it needs that
+			// answer whether or not the mesh was the one who chose it.
+			where, mayAssign := m.MachineSide(l.Port)
+			if mayAssign {
+				at, err := inv.PortFor(ctx, node, m.Module, l.Port, l.Fixed)
+				if err != nil {
+					return nil, fmt.Errorf(
+						"%s needs %d reachable on %s and it could not be assigned: %w",
+						m.Module, l.Port, node, err)
+				}
+				where = at.Machine
 			}
 			if ports[m.Module] == nil {
 				ports[m.Module] = map[int]int{}
 			}
-			ports[m.Module][l.Port] = at.Machine
+			ports[m.Module][l.Port] = where
 		}
 	}