plumbing: format/packfile, share OFS-delta base check

The OFS-delta base-offset predicate landed in three sites in the fix
for the off-by-one: the scanner header decode and the on-demand mmap
object's two delta-resolution paths. Each carried a duplicated copy
of the canonical-Git citation and the same boundary condition.
Consolidate them behind one exported helper, ValidateOFSDeltaBase,
so a future audit reads the canonical condition once.

Assisted-by: Claude Opus 4.7
Signed-off-by: Hidde Beydals <hidde@hhh.computer>