ci: run the differential traversal oracle in the Rust Tests lane (regression from #1644)

[DecisionNerd]

Problem

graphforge-exec's differential_traversal integration test (the fixed-hop reference oracle from #1027) has not run in CI since #1649 (#1644) replaced the Bazel lane.

• The test declares required-features = ["differential-testing"] (crates/graphforge-exec/Cargo.toml). That feature forwards to graphforge-rel/differential-testing.
• No workspace crate enables the feature, so cargo nextest run --workspace skips the target.
• Bazel ran it through a dedicated :graphforge_exec_differential crate built with the feature.

A check of the whole workspace found no other lost coverage. Comparing cargo metadata resolved features with every cfg(feature = …) and required-features gate, differential-testing is the only feature the workspace test build never enables. test-failpoints is still enabled on graphforge-storage through graphforge-api's dev-dependency.

Scope

Run the target in the CI Gate rust-tests lane, for example cargo nextest run --locked -p graphforge-exec --features differential-testing --test differential_traversal, on the same sticky target volume. Add a storage-policy or workflow assertion so the step cannot silently disappear, and add the command to the Rust test gate section in docs/development/agent-environment.md.

Acceptance criteria

• The CI Gate Rust Tests log shows differential_traversal running with a non-zero test count.
• Removing the step fails a repository policy check.
• The local gate docs include the command.

[DecisionNerd]

This issue now natively blocks canonical #1618, without changing its implementation ownership. It is blocked by #1646 so the follow-up starts from the merged current-main Cargo/property-selection policy; no competing source branch is started by this integration session. #1663 is narrowed to historical method/input-digest preservation and completed harness retirement, with no selection changes.

Integration note: if using the proposed supplemental per-package nextest step, the property-overlay selector in #1661 currently requires exactly one nextest invocation. Preserve the mandatory workspace-wide command and its property evidence while recognizing only the intended supplemental differential command, with mutation tests for omission and illegal narrowing. If enabling the feature on the workspace command instead, parse feature-option arguments correctly rather than treating their value as a positional filter. Both paths must preserve default product features, runtime checks, custom harnesses and doctests. The verified frozen review is /tmp/graphforge-1661-frozen-review.38u5az9t/review.md.

[DecisionNerd]

Exact-head acceptance evidence for PR #1665, 5b4d12ac74875cde449789238df8bd6e7bf58d2b:

• Rust Tests log executes python3 scripts/test_environment.py -- cargo nextest run --locked --no-fail-fast -p graphforge-exec --features differential-testing --test differential_traversal: 23 tests across one binary, 23 passed, zero skipped. The primary workspace invocation also passed: 5,149 tests, with 35 existing skipped tests. Custom harnesses and doctests passed.
• python3 scripts/ci/test-ci-storage-policy.py, python3 scripts/ci/property-overlay-contract.py, python3 scripts/ci/test-property-overlay-contract.py, python3 scripts/ci/workflow_policy.py, make gate-registry-check, and bash scripts/ci/test-require-gates.sh all exit0 in an isolated exact-head archive. Independent deletion, feature/package/target, failure-suppression and workspace-narrowing mutations are rejected; removing the supplemental step fails policy.
• docs/development/agent-environment.md contains the supplemental command.
• Exact-head CI Gate is SUCCESS, merge state CLEAN, no unresolved review threads, closing issue exactly ci: run the differential traversal oracle in the Rust Tests lane (regression from #1644) #1662. PR queued using gh pr merge 1665 --squash; queue rerun and landed-main/issue-closure verification remain pending.

Source ownership remains with the external agent; this session coordinates integration only.

[DecisionNerd]

Acceptance outcomes are complete on protected main at 87a7b2d9cfa18bcfba66278414b7176c49284f12 (parent a8843d9a81647d84dcb24336e6b270b01d552ac5). git fetch origin main and git merge-base --is-ancestor 87a7b2d9cfa18bcfba66278414b7176c49284f12 origin/main exit0. git diff --exit-code 87a7b2d9cfa18bcfba66278414b7176c49284f12 5b4d12ac74875cde449789238df8bd6e7bf58d2b -- .github/workflows/test.yml scripts/ci/test-ci-storage-policy.py scripts/ci/property-overlay-contract.py docs/development/agent-environment.md exits0: all four reviewed files are exactly landed.

Merge-group run36666243652, on that exact landed commit, completed SUCCESS with CI Gate success. The PR run and 23/23 actual differential corpus evidence are recorded in comment5903682579. The deletion mutation fails policy and the local gate command is documented.

GitHub currently still reports PR #1665 open with no mergeQueueEntry or merge event, despite the protected main ref containing its exact green merge-group commit. That metadata discrepancy is explicit; this comment does not claim the PR API is merged. The issue is closed by its verified acceptance outcomes and landed work, consistent with AGENTS.md. The source owner keeps branch/worktree ownership and should reconcile the PR record/branch cleanup. No duplicate squash, force-push, competing source edit or extra CI rerun was performed.