Projection pushdown ignores LIMIT BY, ARRAY JOIN and WINDOW, so columns they need get pruned

Bug description

The projection pushdown pass collects which columns a query demands, then prunes the rest from
inner selects. It walks most clauses but not all three of LIMIT BY, ARRAY JOIN, and
WINDOW, so a column referenced only in one of those is invisible to the demand pass and gets
pruned out from under it.

The query then fails at ClickHouse time, with optimizeProjections on by default.

Evidence

Reference counts in posthog/hogql/transforms/projection_pushdown.py on master:

Every clause that participates is referenced twice, once per phase. The three that are missing
are referenced zero times — this reads as three clauses omitted from an otherwise complete list,
not as a deliberate exclusion.

Suggested direction

Visit the three missing clauses in the demand phase, alongside the five already handled.

Notes

Static check on master today: the pass has no code path that could reach those clauses. Not
executed — a runtime repro needs ClickHouse.

Start in posthog/hogql/transforms/projection_pushdown.py by reading the demand phase alongside the existing where, group_by, order_by, having, and prewhere handling. Include LIMIT BY, ARRAY JOIN, and WINDOW in the same investigation, then verify that columns used only by those clauses are retained; runtime validation requires a ClickHouse repro.