68. Release Construction Tracking When an Action Returns Nil

• An action that constructed temporary structs and returned nil skipped cleanup of its construction tracker. Streams therefore held every temporary node. Successful, nil and rejected action results now release tracking references; Go clears the discarded slice tail as well as truncating it. Predicates and bindings use the same cleanup, without changing values held in the environment or returned trees. Generated Node/typed Go, direct typed nested constructors and TypeScript mirror the cleanup; TS uses a finally block.
• For item = "a" -> foldl(nil, list(V{N:1}), (acc,v)=>nil), a discarded stream retained 20,000/60,000 tracked structs before cleanup. A closure diagnostic sampled 8.3 → 24.0 MB of live heap (15 MiB additional retention). The fixed regression on every engine/unit keeps tracking capacity at one; heap growth from 20k to 60k is 0–6 KiB in the recorded run. Returned nodes keep their identity, fields, spans and final-result labeling.
• Six interleaved before/after runs, Apple M3 Max, Go 1.27.1, -benchtime=200ms, 10,000 code-point stream elements: closure B/op 4.20 → 3.89 MB (−7.4%), bytecode/iterative 4.92 → 4.61 MB (−6.3%); 17 fewer allocations per parse. Median time changes are −20%, −12% and −15%. The existing Arith_OperatorsRule batch workload has unchanged allocation volume: engine median time changes are within 2%, generated Node Go is 1.6–4.6% slower and generated typed Go 4.2% slower in this sample. Retain the cleanup for bounded retention; these figures do not claim every action path is faster. TS retained-state/semantic tests pass, but TS timing is not measured here. The full benchmark refresh follows the stream-memory work.
• The baseline is daa364d (code unchanged from 9353e09), compared with the action-reference cleanup in f259b37. Restore eval.go/vm.go and the generated benchmark parsers for the baseline overlay. Reproduce with go test ./internal/engine -run '^$' -bench '^BenchmarkNilActionStream$' -benchmem. Tests cover backing-array references and prefix ownership, stream heap on both units/all engines, generated Go/typed direct rules and TS nil/error paths.