feat: declare an app's storage mounts as a set

[josegonzalez]

dokku_storage_mount gains a mounts list scoped to one process type. state: set makes the list that process type's complete set of mounts and removes any it omits, state: clear removes them all, and present and absent add or remove the listed mounts while keeping the rest, each in a single write. docket export now emits an app's mounts as one state: set task per process type, and the single-mount form now takes process_type into account when deciding whether a mount already exists.

This relies on the per-mount fields storage:mount --replace gained in dokku 0.38.30 (dokku/dokku#9072), which is already docket's minimum version. A process type is the unit of authority because --replace and storage:unmount --all --process-type each act on exactly one, so a recipe managing several process types declares one task per process type. The list form cannot carry a key=value or ro/rw volume option, since --replace would reject or reinterpret it; docket validate refuses one in the list, present and absent fail to plan when another mount of the process type carries one, and export falls back to single-mount tasks for that process type.

The single-mount form previously treated a mount of the same entry at the same path for a different process type as already present, so it was never applied. A recipe that omits process_type for a mount stored under a named process type now plans a change instead of reporting it in sync.

Closes #528