Run global zone binaries for applications that rely on private OS interfaces

Some applications we deploy are strongly coupled to private OS interfaces, and present a challenge for versioning. Some examples include dendrite and maghemite (ddmd will be used as an example for the rest of this issue). For these applications we should be using the versions distributed in the ramdisk inside the application zone to avoid version skew.

Omicron branded zones will mirror /usr from the global zone when installed, so one solution to this is for our application zones to run binaries from a /usr subdirectory, something like /usr/oxide/lib/ddmd. Zone images that Omicron installs will only contain a service manifest (and other optional things like method scripts) that then point to this new location.

Start with brand/src/bin/brand.rs, especially the existing /usr mirroring behavior linked in the issue, and trace how Omicron installs application zones and their service manifests. Define how manifests and optional method scripts should point into /usr/oxide/lib/ddmd so deployed zones use the ramdisk-provided binaries without version skew. Done means the installation path and zone configuration consistently select those binaries.