Slice 1/6 of the postcode_splitter refactor (Hestia-Homes/Model#1100).
Introduces the pure-domain foundation under domain/, with no AWS, Postgres,
or pandas. UserAddress is a frozen dataclass that sanitises its postcode in
__post_init__ via the canonical sanitise_postcode helper, and
iter_postcode_grouped_batches preserves the legacy splitter's batching
invariants (group-by-postcode in insertion order, never split a group,
oversize single-postcode groups dispatched whole, final flush). Updates
UBIQUITOUS_LANGUAGE.md so the User Address term covers both the dataclass
sense (preferred in domain code) and the raw upstream-string sense.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
