Group ignored reasons, values and nonempty properties behind one optional box. Use nonzero backend IDs and serialize fixed role/name kinds on demand, reducing the common AX node from 296 to 96 bytes while preserving payload shape and field order.
Add reproducible CLI and native construction benchmarks. Generated 40000-row text dumps use 382.6 to 341.5 MiB peak RSS; 200000-row text dumps use 1753.5 to 1612.8 MiB. Record the roughly 5.5 percent native construction cost for dense ignored nodes alongside the memory gains.
Validation: cargo fmt --all; workspace clippy with all targets/features and warnings denied; cargo nextest run --no-fail-fast (19890 passed, 17 skipped). All 188 formal benchmark samples match their corresponding before/after outputs.
