[Tomoscorbin]

What

Merges each desired/observed constraint class pair into one class:
PrimaryKeyConstraint and ForeignKeyConstraint now serve both sides, with
name: str | None = field(default=None, compare=False). Deleted:
ObservedPrimaryKeyConstraint, ObservedForeignKeyConstraint, and the
_PrimaryKey / _ForeignKey equality mixins.

Why

constraints.py carried seven entities for two concepts. Four of them existed
to buy exactly one thing — a typed non-optional name at the differ's two drop
sites — and the price was hand-rolled __eq__/__hash__ (generated dataclass
equality is class-strict, and the differ compares desired against observed),
typing-fiction annotations on the mixins, eq=False on all four dataclasses,
and isinstance re-checks in ObservedTable.__post_init__ anyway.

With one class per kind, compare=False gives the documented semantics —
names are creation preferences, not structural identity — straight from
generated equality and hashing.

How the old guarantees are kept

• "An observed constraint carries its catalog name" moves from the type
  system to ObservedTable.__post_init__: a nameless constraint is rejected
  with ValueError, same boundary and same count of runtime checks as the old
  isinstance gate.
• The differ's drop sites (DropPrimaryKey / DropForeignKey take
  name: str) discharge the now-optional name with an explicit
  _require_catalog_name narrow, backed by the Identifier check both drop
  actions already perform at construction.

One deliberate behaviour widening: ObservedTable now accepts any constraint
that carries a name, rather than demanding a distinct class. That is the point
of the merge — the class split never encoded anything beyond name presence.

Nothing changes on the public delta_engine.schema surface; the Observed*
names were exported from domain.model only.

Tests

• test_observed_table_requires_observed_constraint_types reworked into
  test_observed_table_requires_named_constraints (unnamed constraint →
  ValueError), pinning the relocated boundary.
• The class-level "observed constraint requires a name" test is deleted — the
  classes no longer enforce it; the table-level test above owns it now.
• The test builders' as_observed_primary_key / as_observed_foreign_keys
  projections became identity functions, so they are deleted and fixtures pass
  constraints straight through.
• Everything else is a mechanical rename.

Gate: 1400 passed (baseline 1402 — the deleted class-level test was 2
parametrized cases, now covered at the table level), ruff check/format clean,
mypy clean (180 files), all 7 import contracts kept. Net −146 lines.

🤖 Generated with Claude Code