Rebuild as a dialect-independent schema representation

[nbenn]

Summary

• Replaces the R7-era proof of concept with an S7 representation of a relational schema — tables, columns, types, constraints and indexes — together with the protocol that dialect packages implement. The old Remotes: RConsortium/OOP-WG pin resolves to nothing now that R7 has become S7, so the package could not be installed at all.
• Dialects dispatch on a dialect object rather than on a connection class. This package therefore depends on no driver package, and the whole suite runs with no database.
• Rendering a schema is not rendering each table in turn: foreign keys can form cycles, so tables are emitted in dependency order and any foreign key closing a cycle is hoisted into a trailing ALTER TABLE. Engines with no ADD CONSTRAINT opt out through sqlr_defers_constraints().
• Comparison is structural — constraints and indexes are matched on what they constrain rather than on their order or their names — so a schema authored here compares equal to the same schema read back out of a database that picked its own constraint names.
• The PostgreSQL and SQLite dialects live in nbenn/sqlr.postgres and nbenn/sqlr.sqlite, where a render/execute/reflect round trip is asserted against a real engine.

There is no issue to close — this came out of a design discussion, so no Fixes trailer. Say the word if you would rather it hung off one.