Adds Avvir.api.projects.getProjectFloorTradeBuiltElementCounts for the gateway's
new GET /projects/{projectId}/built-elements-by-floor-and-trade.

The as-built IFC export's trade step uses it to disable a trade with nothing built
in the selected areas. The deviation magnitudes it already fetches cannot answer
that: a trade built at its designed position reports a magnitude of zero, exactly
like a trade that was never built.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>