pg-delta: support PostgreSQL temporal constraints (WITHOUT OVERLAPS / PERIOD)

[avallete]

Problem

PostgreSQL 18 introduces temporal constraints such as WITHOUT OVERLAPS and PERIOD. pg-delta does not appear to model or diff this metadata today, which can cause false "no change" results when constraints differ only by temporal semantics.

Current gaps

• Temporal metadata is not extracted from catalog sources such as pg_constraint.
• The table-constraint property schema does not include temporal fields, so exported/introspected state cannot represent temporal PK/FK accurately.
• Constraint diffing compares scalar fields and column lists but not temporal metadata or an equivalent full rendered definition, so regular <-> temporal transitions may be treated as unchanged.
• The integration test matrix does not currently cover PostgreSQL 18 temporal DDL.

Desired behavior

• Safely extract and represent temporal constraint metadata in the constraint model.
• Treat non-temporal <-> temporal changes as drop/recreate (or another explicit migration), not as a no-op.
• Add PostgreSQL 18 integration coverage for temporal PK/FK roundtrips.

Suggested work

1. Extend catalog extraction to read temporal metadata in a PG-version-safe way.
2. Thread the new field through the table constraint model/schema.
3. Update diffing so temporal vs non-temporal constraints are recognized as changes.
4. Add PostgreSQL 18 to the relevant integration matrix and cover temporal primary key / foreign key roundtrips.

[avallete]

Extra reproduction detail from the benchmark notes: besides create-time temporal PK/FK coverage, there is a useful ALTER transition path on two related tables (contacts, conversations) that drops and re-adds the PK and FK to introduce WITHOUT OVERLAPS on the PK and PERIOD on the FK columns. That is the scenario that historically behaved like a silent downgrade when temporal metadata was not modeled, so it would be good to confirm the integration coverage in #186 exercises that transition as well. For context only: pgschema #364 / #365.