CI: bump upload-artifact (v4->v6) and download-artifact (v4->v7) to Node 24

Problem

Follow-up to #36085 (deferred there). actions/upload-artifact@v4 and actions/download-artifact@v4 run on Node 20 (deprecated; runners default to Node 24 on 2026-06-16). Their Node-24 releases are multi-major jumps, so they were split out for a reviewed bump.

Verified safe to bump

• upload-artifact v4 → v6 — inputs identical (name, path, if-no-files-found, retention-days, compression-level, overwrite, include-hidden-files); only change is Node 24 + min Actions Runner 2.327.1 (our hosted ubuntu-24.04 runners satisfy this).
• download-artifact v4 → v7 — inputs identical. The only behavioral break (v5) affects downloads by artifact-ids; every usage in this repo downloads by name or pattern/merge-multiple, never artifact-ids, so the migration guide's "no action needed" applies. (v7, not v8 — v8's no-unzip change is avoided.)

A FORCE_JAVASCRIPT_ACTIONS_TO_NODE24 smoke-test already confirmed the existing v4 artifact actions run fine on Node 24, so this is not a cutover risk — it clears the deprecation warning.

Search the repository's GitHub Actions workflow files for upload-artifact@v4 and download-artifact@v4, then review each usage for the inputs and download patterns described in the issue. Update the action versions to v6 and v7, confirm no usage relies on artifact-ids, and verify the workflow changes pass CI.