fix(params): stop accepting null for a comparison parameter

[tiagolauer]

Fixes #299.

The bug

// schema: email: string | null
Params<DB, 'select id from users where email = $1'>  // [string | null]
db.query('select id from users where email = $1', null); // compiles, matches nothing

Join-induced nullability leaks the same way, and shows the mechanism plainly — the | null belongs to the result side (a missing match makes the column null in the output), not to the value you compare against:

Params<DB, 'select u.id from users u left join posts p on u.id = p.user_id where p.views = $1'>
// [number | null]

The fix

ParamType resolves through ResolveColumnLoose, which reuses result-column resolution verbatim. Comparison parameters are now NonNullable, with three carve-outs where null is meaningful:

• is distinct from — comparing against null is the entire point of it.
• SET assignments and VALUES tuples — storing null is ordinary. The scan keeps only the two previous tokens, so set a = $1, b = $2 gives the second placeholder no local hint that it is still a write; the region is carried along the scan instead.
• an unresolved column — stays unknown. NonNullable<unknown> is {}, which would have rejected the null such a slot is still allowed to take. This one was caught by an existing test (tests/update-where-subquery.test-d.ts) going red mid-change, not by foresight.

Verification

tests/nullable-params.test-d.ts, eleven cases. Four red on master:

tests/nullable-params.test-d.ts(16,3): error TS2344: Type 'false' does not satisfy the constraint 'true'.
tests/nullable-params.test-d.ts(20,3): ...
tests/nullable-params.test-d.ts(26,3): ...
tests/nullable-params.test-d.ts(36,3): ...

• =, like, in, and the LEFT JOIN column all drop the null
• carve-outs pinned: is distinct from, update ... set, every assignment in a multi-column SET, and insert ... values
• controls: a non-nullable column is unchanged, = any($1) still wraps to number[], limit $1 is still number

tsc --noEmit clean, 192 runtime tests pass. Budget 193,992 (+1,506).