The concurrency cap is the mouse

The county's fee calculator is a Qt app from 2011. No API, no source, and every online permit needs its number.

You put the app on display-1 and write one exec script at /fees/[parcel]. The handler POSTs an input/batch to the display: click the parcel field, type the number, press Return, select the total, ctrl+c. Then it GETs /api/v1/display/clipboard and returns {"fee": 412.50}. Above the handler sits // @concurrent false. There is one cursor, so there is one lane; callers are serialized, and the queue depth shows up in the stats API.

The permit portal now GETs a fee from a desktop app that never learned HTTP. Nobody rewrote it. Nobody licensed a second copy.

The concurrency cap is the mouse.