The seat set was a Go slice compiled into the controller and referenced by
name everywhere, so changing it meant a rebuild and a freeze-prone deploy. It
is now a table: catalogue keeps the shipped set as defaultSeats (the seed and
the fallback) and a loadable working set; inventory adds the seat table
(migration 0034), Seats to read it, and SeedSeats to fill it idempotently
without overwriting an operator's edit; migrate seeds it; openInventory loads
it, and an empty or unreadable table leaves the compiled defaults in force so
it can never brick the control plane's boot.
Behaviour-neutral: the seeded table equals the defaults. Phase 2 (reference by
a stable id so a rename touches no manifest or code, and the builder reads the
set from the mesh) follows.
