37. No Intermediate Lists In concat

• concat(list($first), map($rest, (r) => r.x)), the usual way to build a list from a first element and the rest, built two lists only for concat to copy their elements: about 19k list nodes per JSON parse. When an argument of concat is itself a call of list, map or concat, the closure backend now pushes that call's elements directly onto the stack concat collects on, and the code generator emits the same pushes. This is done only when every argument is such a call (the common case): with any other argument, concat checks it only after evaluating all of them, and fusing would report a different error when two arguments fail. The VMs still build the intermediate lists (fusing them there would take new instructions).
• Effect (min of 12 interleaved runs, Apple M3 Max): JSON 14.4 → 14.1 ms (closure), 10.0 → 9.8 ms (generated), 21.4 → 20.0 MB per parse; CSV 6.6 → 6.2 ms (closure), 4.8 → 4.5 ms (generated), 15.6 → 14.0 MB; minilang unchanged within noise.