Migrate authoritative OSTW reference evidence from Wright

[Teakowa]

Goal

Make deltin-rs the durable owner of the pinned OSTW reference evidence and reproducibility infrastructure that still remains under wrightkit/wright/compatibility/ostw/.

Context

deltin-rs already owns DEL/OSTW-compatible language semantics and defines compatibility against a pinned upstream reference. Its current compatibility contract treats upstream differential comparison as gap-discovery evidence rather than a default CI merge gate.

The repository split is incomplete at the evidence layer: Wright still contains the OSTW v3.4.0 reference metadata, explicit-root corpus/probes, recorded observations/results, reconstruction evidence, and the legacy oracle runner. Wright has already removed the live OSTW reference replay from its default CI, but deleting this tree before an owner-side migration would create an evidence/reproducibility gap.

Scope

• Identify which artifacts under wrightkit/wright/compatibility/ostw/ are authoritative DEL/OSTW language evidence versus Wright-specific consumer/integration snapshots.
• Move the authoritative pinned-reference identity, corpus/probes, recorded results, provenance, and reproducibility tooling into deltin-rs.
• Preserve immutable reference identities, hashes, corpus roots, provenance, and declared observation semantics during the migration.
• Integrate the migrated evidence with docs/compatibility.md, docs/provenance.md, and the support-matrix evidence model instead of creating a second compatibility contract.
• Provide a documented maintainer workflow for reproducing or intentionally refreshing pinned upstream observations without requiring the upstream runtime on every PR.
• If the existing runner is retained, bound external-process/network waits and expose enough progress to diagnose stalls.
• Define the exact residual evidence, if any, that Wright must keep for its current migration/provider integration tests so the Wright follow-up can remove the remaining owner-style OSTW harness.

Non-goals

• Running the live upstream OSTW compiler as a required merge gate on every deltin-rs PR.
• Copying, translating, or redistributing unlicensed upstream compiler implementation internals.
• Requiring output-text identity when observable semantics and declared contracts do not depend on it.
• Moving Wright-only tooling, LPP, distribution, or product regression tests into deltin-rs.
• Expanding DEL/OSTW support merely because additional upstream behavior is observable during migration.

Acceptance criteria

• deltin-rs can reproduce or validate its declared pinned OSTW reference evidence without depending on files in the Wright repository.
• The authoritative upstream reference identity and provenance are recorded once in the deltin-rs evidence/documentation model.
• Migrated corpus/probe/results data retain their immutable identities and can be traced to the Wright evidence they replace.
• The owner-side workflow distinguishes reference reproduction/gap discovery from normal native CI merge gates.
• No dependency from deltin-rs back to Wright tooling internals is introduced.
• Documentation states which evidence remains authoritative in deltin-rs and which Wright snapshots, if any, are consumer-only.
• A Wright follow-up can safely remove compatibility/ostw/ owner infrastructure without losing declared compatibility evidence.

Dependencies

• Wright ADR-0009 language ownership boundary.
• Wright PR #177 removed the legacy live OSTW reference baseline from default Wright CI.

Planning notes

Treat the existing Wright tree as migration evidence, not architecture to preserve mechanically. Keep only observable/reference contracts that remain useful to deltin-rs; do not reproduce legacy harness complexity without evidence that it is needed.