Admission derived the live-entry count as `HLEN - 3`, where 3 was the number
of `meta:*` fields written 35 lines earlier in initialize(). Nothing tied the
two together, so adding a fourth meta field would shift the capacity ceiling
by one.
Name the offset `META_FIELD_COUNT` next to initialize(), and add a guard test
asserting a freshly initialized ledger holds exactly those three fields, plus
one pinning that a hard_limit of N admits exactly N reservations.
References #4575
Co-authored-by: icn5381 <255778606+icn5381@users.noreply.github.com>
