Per-area defaults for /etc/whp/privd.json "areas" (prep seams fix item 6).

  configs/privd.d/<area>.json          one JSON object: the default of areas.<area>
  configs/privd.d/<area>.<lane>.json   keys of areas.<area> another lane owns (ruling P37);
                                       a key may be defined by one file only

Only the <area> lane edits its file. The area declares the section's Go
type in panel/internal/privd/ops/<pkg>/register.go ConfigSection(); a
default for an area without one, or with a field the type lacks, fails
TestShippedPrivdDefaults_DecodeWithTheirSection (and privd refuses to start).

ensure-whp-privd.sh runs `whp privd-config merge-areas --dir configs/privd.d`
on every update: keys missing from privd.json are added (recursively into
objects); a value already there is never changed, so an operator's edit wins.
Changing a shipped default therefore does not change hosts that already
have the key: a lane that must change a live value needs a migration step.

Never remove or rename a key without a migration: privd.json keeps the
merged value, and the strict decoder then refuses privd startup on every
host that has it (fix round 1 M5).
