Add sqlr_test_types() to round-trip every type in each dialect

[nbenn]

Summary

• The dialects' round-trip tests list their types by hand, so a type nobody listed goes unexercised. Core now ships the list as sqlr_conformance_types(), one entry per type class and per value of each parameter it carries, and a core test fails when the IR gains a class or parameter the list does not cover.
• A dialect's test suite calls sqlr_test_types() with a function that opens a connection. Each entry becomes its own test, which renders a one-column table, creates it, reflects it and fails with the sqlr_diff() lines unless nothing changed. Types the dialect cannot represent are declared in skip with a reason and skipped citing it; an unknown name is an error.
• Core's suite has no database, so the round trip itself runs downstream. Run locally against both dialects' current main, it finds the 8 SQLite renderer losses listed in the issue (tinyint passes since Reflect column types with this dialect's own mapping sqlr.sqlite#2) and 6 on Postgres, where tinyint does not render, integer_unsigned reads back as integer, numeric_precision with scale 0, char_unsized as char(1), and varbinary and binary as an unbounded bytea. Both dialects adopt the helper in their own PRs once this is on main, since they install sqlr from there.
• DBI joins Suggests, for the helper only.

Fixes #10