fix(ci): unblock R10 post-merge verification for stable release

[masarray]

Scope

Release-blocker-only follow-up on the physically proven R10 baseline.

• allow the first-party R10 post-merge workflow through the clean-room identifier allowlist;
• checkout the immutable ARIEC61850 R10 engine into the sibling path already required by the ARSAS project references;
• keep engine lock 648124097621046f5f127ceb1cf853fea54db730 unchanged;
• no application/runtime/SCL/MMS/reporting/discovery changes;
• PR perf(scl): reuse short-lived save enrichment snapshot #327 is intentionally excluded.

Acceptance

Merge only after exact-head Build ARSAS and applicable guards are green. After merge, require Smart Discovery Post-Merge Production Verification on main to pass before creating the stable release.

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[masarray]

Closing as superseded by the accepted v1.6.40 mainline/release path. The source-clean allowlist and immutable R10 engine checkout required by this historical v1.6.38 CI patch are present in current main, and the post-merge Smart Discovery verification passed on the accepted v1.6.40 lineage. Do not merge this old branch into main: it predates the production-route correction and current release. This is PR cleanup only; it does not alter or republish v1.6.40.