pg_class OIDs collide for names sharing first 3 chars and length #93

The canonical table-identity OID formula reads only the first three characters and the length:

((unicode(c1)*1000000) + (unicode(c2)*10000) + (unicode(c3)*100) + (len*7)) % 1000000 + 16384

Any two relations sharing those produce one OID. Demonstrated:

OID is the join key from pg_class to pg_attribute, pg_index, pg_constraint, and pg_depend, so an ORM introspecting orders can receive orderz's columns.

Pre-existing, not introduced by #87. Fixing it means one canonical low-collision function shared by every view and Rust site, plus a data migration rewriting persisted OIDs in pg_constraint.conrelid, pg_index.indrelid, pg_attrdef.adrelid, and pg_depend.objid/refobjid.

Related: oid_hash (src/functions/hash_functions.rs) uses Rust's DefaultHasher, which is not guaranteed stable across Rust releases, so OIDs derived from it could change on rebuild. It should be retired in the same work.