Compatibility experiment, not proposed grammar. probe.rs imports only the matching existing oa_core rlib; it performs no production or Cargo edits.

In a coordinated direct-rustc writer slot:

. .local/toolchain/env.ps1
rustc --edition 2024 -D warnings --crate-name constructor_defaults_probe harness/experiments/constructor-defaults-20260911/probe.rs --extern oa_core=.local/target-gnullvm/debug/deps/liboa_core-3e81c616fea1d901.rlib -L dependency=.local/target-gnullvm/debug/deps -C linker=clang.exe -o .local/constructor-defaults-20260911/probe.exe
& .local/constructor-defaults-20260911/probe.exe NEW_ISOLATED_OUTPUT_DIRECTORY

Use a fresh output directory when reproducing so the committed evidence is not overwritten. The rlib SHA256 is 5018A1FD7286ED9DFEDCBBF5B68B399AD5D143F2A9AAAED3078CFC934C9589C4. The exact compiler, core source hashes, executable hash and result inventory are in provenance.json.

results/results.tsv records 54 admitted explicit/omitted cases and 12 missing-required-field refusals, covering both OA and SA source 7. Every admitted occurrence is compared with a literal expected typed body and the full explicit-case Project. It also requires exact source retention on stabilization/no-op, a name-only patch retaining omissions, and canonical format/reparse equality. Both scenario viewpoint omission cases separately require MixedLegacy equality and canonical stability.

The .canonical.arcadia siblings intentionally expand hydrated defaults; they are not expected to retain authored omission. Exchange operation none is deliberately omitted by canonical serialization. Files named omit-bindings, omit-kind, omit-operator, omit-branches, omit-guard, or omit-occurrences are expected invalid fixtures, retained to show the specific missing-field diagnostic. They are not modeling examples.

The program/result files are durable evidence. The compiled executable and initial/final build logs remain in ignored .local/constructor-defaults-20260911/. Other constructor families and old schema versions were not exhaustively tested.