Raise genesis's bus where the nats module keeps it, so adopting it keeps it (hq issue 590, ADR 0333)

The bundle raised the bus as mesh-broker on docker volumes; the catalogue's nats module keeps it as
mesh-broker-nats on directories of the machine, so the broker step refused rather than raise a second
bus, and genesis stopped at step 18 of 24.

The bundle now raises mesh-broker-nats with its configuration and user list in /var/lib/nats-module/conf,
its streams in /var/lib/mesh-broker-nats and its certificate in /var/lib/mesh-broker-tls, published on the
node's bus port. The broker step holds that layout to the module's manifest field by field before
registering anything, says the one restart onto the module's image, and waits until the bus runs that image
and its monitor answers. The installer's bridge writes the composed users at the module's bus-users path and
signals mesh-broker-nats. The bundle's bus configuration and user list are seeds, so a re-run after the
adoption does not put the bundle's one user back over the module's.

examples/README.md

@@ -12,7 +12,8 @@ mesh runs on:
 2  a database per context        `inventory`, `identity` and `licences`
 3  those contexts' schemas       mesh-controller migrate
 4  the bus's certificate         mesh-controller broker certificate
-5  the bus runs                  NATS, with the controller's account as its first user
+5  the bus runs                  NATS as mesh-broker-nats, where the nats module keeps it
+                                 (ADR 0333), with the controller's account as its first user
 6  the control plane runs        mesh-controller serve, given the bus (MESH_BUS_NATS)
 ```
 
@@ -26,6 +27,12 @@ rootsecrets.go). A template whose bus connection it does not recognise is refuse
 naming the placeholder it looked for. The bundle that raised the previous broker was deleted with
 ADR 0331; there is one bundle, and every lab bed names it.
 
+**The bus is raised where the `nats` module keeps it** (novox/hq ADR 0333): the container
+`mesh-broker-nats`, its configuration and user list in `/var/lib/nats-module/conf`, its streams in
+`/var/lib/mesh-broker-nats`, its certificate in `/var/lib/mesh-broker-tls`, published on the node's bus
+port. At the installer's broker step the module adopts it: its own image replaces the upstream one, once,
+on the same streams, users and certificate. `internal/bootstrap` holds the bundle to the module's manifest.
+
 Build an installer carrying a builder, and point it at this:
 
 ```