Splits the non-destructive validation phase out of update_partition_table()
into a dedicated method. update_partition_table() now reads top-down as
"check buffer state, find running app, validate the new table + plan the
target slot, then commit", with the destructive write isolated to the
final block.
The chosen slot and optional copy-source partition are returned via a
small PartitionTablePlan struct so the caller no longer juggles the two
candidate-index variables. Refactor only; behaviour and error semantics
are unchanged.
J. Nick Koston
4490bbf23a