Separate coverage job

[maxtropets]

As per #7897

CI: keep coverage on a separate, less frequent schedule

Companion to the ci.yml reorg that drops the coverage duplicate from PR
runs. Coverage is still useful, just not on every PR, so this adds a
dedicated `coverage.yml` workflow that runs on the same triggers as the
existing benchmarks workflow:

  - cron "0 8,20 * * 1-5"  (twice a day, Mon-Fri)
  - push to main
  - workflow_dispatch       (manual trigger)

The job is the old VMSS-C steps lifted verbatim: setup-ci, debug build
with -DCOVERAGE=ON, unit tests, full e2e bucket
(-LE benchmark|suite|unit), scripts/coverage.sh, and the same
`coverage-report-html` artifact upload + `$GITHUB_STEP_SUMMARY` write.
Runs on the same gha-vmss-d16av5-ci pool with the same caps, and uses
the same pinned action SHAs as the rest of the repo.

Net effect: PRs are ~32% faster to validate, coverage still gets
generated regularly and on demand, and the artifact name (and consumers
of it) are unchanged.

[Copilot]

Pull request overview

Adds a dedicated GitHub Actions workflow for generating coverage outside PR CI, preserving the existing coverage build/test/report sequence on scheduled, main-branch push, and manual triggers.

Changes:

• Adds coverage.yml with benchmark-like triggers.
• Runs coverage build, unit/e2e tests, coverage report generation, summary writing, and artifact uploads.

Custom instructions used

• .github/copilot-instructions.md