paryplanner: set_partition_uuids` eaks `partition_plan` into the caller's scope

[jmt-lab]

[P2] set_partition_uuids (L573-594) leaks partition_plan into the caller's scope — missing local. (commenting near the UKI typecode change since L579 is not in the diff)

set_partition_uuids() {
  local -n pp_uuid
  pp_uuid="${1:?}"
  partition_plan="${2:?}"   # <-- L579, missing `local`

The sibling set_partition_sizes correctly declares local ... partition_plan (L235). Copy-paste drift.

Today the only callers use uppercase PARTITION_PLAN, so no collision, but any future caller (e.g. a helper inside img2img) with its own lowercase local partition_plan will find its value silently clobbered by this function via bash's dynamic scoping.

Fix: local partition_plan="${2:?}" on L579.

Originally posted by @jmt-lab in #701 (comment)