Add a hectic.immutable parent table. Tables inheriting it get auto-attached
BEFORE INSERT/UPDATE/DELETE/TRUNCATE row+statement triggers that block DML
unless the session sets hectic.migration_mode='on' (intended use: SET LOCAL
inside a migration transaction). Same exemptions as the rest of the bundle
apply (hectic schema, partitions, temp tables, GUC-excluded schemas).
database diff now appends an --- IMMUTABLE TABLE DATA --- section to its
output, with per-table unified row diffs of every table inheriting
hectic.immutable, surfacing drift in 'frozen' reference data alongside schema
drift. Subcommand exits non-zero when either schema or data differs.
Test postgres-init-hectic-inheritance extended to 10 cases covering
immutable triggers, DML blocked outside migration_mode, SET LOCAL allowing
DML inside a transaction, GUC not leaking past COMMIT, and TRUNCATE under
migration_mode.
