fix(parse): let a shadowing name replace the table it hides

[tiagolauer]

Fixes #285. Fixes #286.

Two issues, one root cause and one helper, which is why they are together here rather than in two PRs that would each need half of it.

The bug

Object intersection is additive: DB['users'] & { id: number } still exposes name. Two places relied on it where SQL says the name is hidden.

A CTE was omitted from the schema only for the outer query, so a later CTE body still saw the real table:

StrictQuery<DB, 'with users as (select id from users), t as (select name from users) select name from t'>
// was: { name: string }[]   -- inside `t`, `users` is the CTE, which has no `name`

And a derived table whose alias reuses a real table name was intersected with it:

StrictQuery<DB, 'select name from (select id from users) users'>
// was: { name: string }[]   -- the alias hides the table; only `id` is in scope

Both cases have strict mode accepting a query the database rejects, and typing a column the query cannot produce. The comment at ResolveCteContext already described the first gap.

The fix

ShadowedBy pairs the existing OmitShadowedTables with the intersection, and both sites use it.

It is gated on the overlay being non-empty. That matters: the omit is a mapped type over every table in the schema, and running it for the empty overlay a plain query carries measured +18,744 instantiations, 10% of the whole budget and blew the ceiling. With the gate the budget lands at 192,086, slightly under master's 192,486, because the intersection it replaces used to run on every query.

Versioning

Minor: strict mode reports a mistake it previously let through.

Verification

tests/shadowing.test-d.ts, two cases red on master (one per issue), seven controls green throughout:

• outer-query CTE shadowing still works (it always did)
• a CTE referring to an earlier CTE
• an unshadowed table still visible from a CTE body
• a derived table with a fresh alias
• a derived alias still exposing its own columns
• a plain query

tsc --noEmit clean, 192 unit tests pass.

Found during an audit pass; fix and tests written with Claude Code.