ci(maintenance): use pinned ARIEC61850 in RCB export guard

[masarray]

Purpose

Make the Legacy SAS RCB export guard deterministic and consistent with the application engine dependency, rather than cloning the moving upstream main branch.

Root cause and change

The existing guard read the engine from masarray/ARIEC61850@main, while ARSAS builds against engines/ARIEC61850.lock.json. A later unrelated engine commit could change this guard's result for an unchanged ARSAS candidate.

• Validate the integration lock's repository and exact 40-character commit.
• Fetch/checkout that immutable engine commit and reject a mismatch or Git command failure.
• Add the engine lock to both push and PR path filters so an engine-pin change invokes the guard.
• Preserve the existing RCB export source inspection and result artifact without touching product code.

Preflight and scope

Base: 9d081f54fc6466d13c099e8a94a020c487a36703; head: 2dd0c8d20b88ee6a5f6b43688c646c24a117b6d9. Exactly one workflow YAML changed, 20 additions / 2 deletions. Verified every one of the six engine source files and every required engine-source text contract currently checked by scripts/verify-rcb-export.ps1 is present at the locked ARIEC61850 commit 648124097621046f5f127ceb1cf853fea54db730. The script itself, app source, engine lock, reporting and release paths are unchanged.

Validation

• Actual one-file commit diff and remote blob inspected.
• Require the RCB export guard and Windows Build ARSAS to complete successfully on this PR's exact candidate; CI is pending at PR creation, not claimed passed.
• No physical IED operation, new vendor-specific logic, or modification of published v1.6.40.
• DCO sign-off remains a maintainer/contributor gate; do not auto-merge.

This is separate from maintainability PRs #413 and #414 and SCL PR #374. Tracks #380.

[masarray]

Combined with #413 and #414 in draft integration PR #416 (#416). Its selected-RCB guard passed with the locked engine SHA, and the entire combined head passed 9/9 workflows. Keep this source PR open for traceability; avoid separate merge if #416 becomes the signed-off integration vehicle.

[masarray]

Superseded by consolidated PR #416, merged to main as e7b43508fb3b2bf5fbda7be8ed8425da74bb9766. Verified that this PR's exact changed-file blob SHAs are present in #416's merged tree; no separate merge of this source PR is necessary. Closing this draft to prevent duplicate integration. This does not imply any new physical IED acceptance or release promotion.