27. Struct and Call Operands on the Expression Stack in the Closure Backend

• The closure backend's evaluator built two slices for every T{...} (field names and values) and one for every built-in call (arguments). Values and arguments now go on the parser's expression stack (parser.estack, as in the VMs since change 14) and field names into a reused buffer; newStruct and the built-ins retain neither.
• Effect (min of 8 interleaved runs, Apple M3 Max, closure backend): Arith_OperatorsRule 11.1 → 10.6 ms, 40.6k → 0.6k allocations, 14.2 → 12.2 MB; XML 38.9k → 19.1k allocations; minilang 8.3k → 1.1k allocations; times otherwise within noise. All backends now allocate fewer than 1.5k objects per parse on JSON, CSV, Operators Rule and minilang; XML's remaining 19k are text(...) results (a string stored in an interface needs a heap header).