Add union() and unionAll() to QueryGenerator

[danielbeardsley]

Summary

Adds union() and unionAll() operators to QueryGenerator for combining SELECT statements. Operands may be another QueryGenerator (built and validated immediately) or a raw SQL string with params.

$qGen->select('id')->from('orders')->where('total > ?', 100);
$qGen->union($archivedQGen);
$qGen->unionAll('SELECT id FROM pending_orders');
$qGen->order('id');

Design notes

• Implemented as real methods rather than via __call, because the existing nested-generator path builds sub-queries with skipClauses: true, which strips SELECT/FROM keywords — correct for WHERE fragments, wrong for union operands that must render as complete SELECTs.
• The UNION / UNION ALL keyword is baked into each stored piece (glue stays "\n"), so the two can be mixed freely in call order.
• The clause renders between HAVING and ORDER BY, so a trailing order()/limit() applies to the whole union result (MySQL semantics) and params merge in placeholder order.
• QueryGenerator operands are validated at union() call time; opt out with skipValidation() on the operand.

qa_req 0 -- CI has new tests

CC @sterlinghirsh @sctice-ifixit

🤖 Generated with Claude Code

[djmetzle]

Neat. CR 🥢

[sctice-ifixit]

I was worried about param ordering, but testUnionParamOrdering covers it. According to Claude: putting union between having and order in $methods means build() emits the union params in the right placeholder position, and the operand's own build() orders its params internally, so nesting works too.

One thing outside the diff that sent Claude down the wrong path: build()'s return annotation says the params come back as array<string, mixed>, which had it worried about named params colliding between the two halves of a union. As far as I can tell nothing in PopSQL uses named params (every example and test is positional ?) and build() returns a list, so it seems like that should be list<mixed>. Apparently Psalm sees this but emits it as Info rather than Error.

[sctice-ifixit]

CR 🌷