v0.5.0

Install: pip install oxihipo==0.5.0

Added

• Chain::bank_occupancy — which banks carry data, in how many events, and
  how many rows, without inflating a single bank or column. Every number is a
  function of a bank's per-event byte extent, which both columnar layouts already
  record in their bank-offset tables, so on Lz4PerBank and Lz4PerColumn
  nothing beyond each record's header and offset tables is decompressed. Honors
  the chain filter, takes an optional global-index range, and follows the usual
  threads convention (0 = all cores, 1 = sequential).

  This exists because the operation is easy to get wrong outside the library.
  Computing it from Chain::events costs an OwnedEvent — a copy of every
  event's bytes — and enumerating a per-column event's structures first
  synthesises a whole event out of separate column streams: measured at
  19 µs/event on Lz4PerBank and 26 µs/event on Lz4PerColumn in a downstream
  tool, against roughly 1.5 µs/event for reading the presence tables.

  EventCtx looks like the escape route and is not: it avoids the copy but
  cannot enumerate a per-column record's banks, because that needs exactly the
  synthesis it exists to avoid. A caller who tried it got 4 banks out of 71 —
  fast, plausible, and wrong with no error. Putting the operation here, with a
  cross-format equality test, is the fix for that class of mistake rather than
  for one instance of it.

  Banks declared but never populated are returned with zero counts, so "never
  written" stays distinguishable from "not in the dictionary". A bank opened with
  no rows counts as carrying no data, which is the question being asked.

  Classic layouts (None/Lz4/Lz4Best/Gzip) keep no per-bank table, so
  their records are decompressed and their events walked — but with no per-event
  allocation, so they gain too.

• BankOccupancy in the crate root, the per-bank result type.