Release v0.4.0

Storage migration: the translation-catalog substrate moves from ETS to
persistent_term. The benchmark proved persistent_term reads roughly 55%
faster for this read-hot / load-once library because persistent_term:get/2
returns the term without copying it onto the caller's heap. The public API and
all lookup/fallback/idempotency semantics are unchanged; the only observable
differences are documented below. The minor bump follows the 0.x SemVer
policy above.

Changed

• Catalog storage migrated from ETS to persistent_term (new module
  erli18n_pt_store). Each {Domain, Locale} catalog is one persistent term
  (key {erli18n_catalog, Domain, Locale}) holding a single map of its entries
  plus the header. Reads are copy-free and lock-free from the calling process.
  There is no lookup behaviour change: lookup_singular/4,
  lookup_plural_form/5 and lookup_header/2 keep their exact specs, guards,
  miss semantics (undefined) and return shapes.
• reload/3,4 and unload/2 now trigger a node-wide persistent_term
  literal-area garbage collection. Replacing or erasing the catalog map
  defers a cleanup in which every process still referencing the old map runs a
  major (fullsweep) GC and all processes are made runnable to scan their heaps.
  This cost is paid once per (re)load or unload and is negligible for erli18n's
  load-once-at-boot workload, but it is a real cost the previous ETS storage did
  not have. It is documented here and in the erli18n_server /
  erli18n_pt_store module docs, never hidden.
• memory_info/0 — the ets_bytes field now reports the approximate
  persistent_term storage size in bytes. The field name is kept for backwards
  compatibility with the 0.3.0 return shape (the storage is no longer ETS).
• A lookup against a stopped catalog now returns undefined instead of
  crashing. Because the catalogs live in runtime-owned persistent_term
  rather than in a process-owned ETS table, a missing or unloaded catalog is a
  clean miss on the read path, not an access to a dead table.

Removed

• erli18n_table_owner and the entire ETS heir / 'ETS-TRANSFER' /
  give_away/3 handoff subsystem. That machinery existed only so ETS catalogs
  survived a worker crash (Finding #10). persistent_term is node-global and
  runtime-owned, so a worker crash destroys nothing: the supervisor collapses to
  a single erli18n_server child under one_for_one (was rest_for_one with
  an owner-first ordering), and the secondary ETS catalog index and the
  associated erli18n.hrl macros are gone with it.