Run one builder suite on every dialect; cover the Postgres entry

[Makisuo]

Stacked on #4.

Borrowed from how drizzle tests its dialects: one shared suite run against every database, with named, reasoned skips. On top of that, the coverage manifest we already had for ClickHouse now also covers the shared builder and the Postgres entry.

Shared builder suite (tests/core-cases.ts)

• 20 cases cover selection, every shared operator, params, group/having, limit/offset, all six join kinds, CTEs, unions, route/crossTenant and format.
• Both databases read the same fixture rows from a WITH (values(...) on ClickHouse, VALUES on Postgres), so the suite writes nothing.
• It runs live on ClickHouse under both join_use_nulls settings, and on PGlite on every vitest run. core-sql.test.ts snapshots the SQL and parameters for each dialect.
• Real differences between the databases are recorded per target: / is integer division on Postgres, and ClickHouse fills a missing join row with defaults when join_use_nulls=0. rejects covers clauses a dialect refuses (format on Postgres).

Manifests (tests/dialect-coverage.test.ts)

• Every query/union method, expression/condition operator and param kind needs a core case.
• Every ./postgres export needs a case in dialect-cases.postgres.ts, or an exemption with a reason.

Bugs it found, fixed in the first commit

• Postgres: a UNION ALL branch with its own WITH/ORDER BY/LIMIT was a syntax error. Branches are now parenthesized (clauses.parenthesizeUnionBranches).
• Postgres: param.float compared with an int8 column was bound as int8, so 19.5 was rejected. A param.bool in a select list was bound as text. Float, bool and timestamp params now bind with a cast. string/int stay uncast so they still compare with enum and int4 columns.
• Docs: custom("int8", Schema.String) for exact int8 fails on drivers that send bigint (PGlite, postgres.js).

Verified: typecheck; bun run test (427 passed, live-only suites skipped); the full suite against ClickHouse 26.8.2.7 in Docker (593 passed, 0 skipped).

🤖 Generated with Claude Code

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