Update Downstream.yml to use codecov-action v4

[ChrisRackauckas-Claude]

Summary

PR #43 attempted to bump codecov/codecov-action from v3 to v4 but is now stale with merge conflicts. The repository structure has changed significantly since then, but Downstream.yml still uses codecov-action v3.

Background

• codecov-action v4 requires a token (tokenless uploading is no longer supported)
• The shared workflow in Tests.yml handles codecov internally
• Documentation.yml now uses a shared workflow

Proposed Change

Update .github/workflows/Downstream.yml line 53-55 from:

- uses: codecov/codecov-action@v3
  with:
    file: lcov.info

to:

- uses: codecov/codecov-action@v4
  with:
    file: lcov.info
    token: \${{ secrets.CODECOV_TOKEN }}
    fail_ci_if_error: true

Prerequisites

Ensure CODECOV_TOKEN is configured in the repository secrets.

Related

• Stale PR: build(deps): bump codecov/codecov-action from 3 to 4 #43

cc @ChrisRackauckas

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