Add a proper build step (dummy `build` is a temporary CI stopgap)

[Shinrai]

What / why

The @v4 CI coverage-badge leg runs coverage_build_command (default npm run build) before computing coverage. This repo has no build step, so the PR Coverage Badge check failed on release PR #17 with npm error Missing script: "build". Coverage itself is fine — real ci:coverage + a json-summary reporter — so the badge stays enabled.

Stopgap in place (commit baa490d on next)

A dummy no-op build so npm run build exits 0:

"build": "echo 'no build step - stopgap for CI coverage-badge; see tracking issue'"

This is temporary.

The real work — give this repo a proper build

Replace the dummy with a genuine build step for this package, then drop the stopgap. If, after review, fix-headers is deliberately source-only with no build, the clean alternative is skip_build: true in the workflow-ci.yml caller (the designed no-build mechanism) rather than carrying a dummy script. Either way, remove the dummy build.

Not an upstream issue

CLDMV/.github's @v4 already honors skip_build and coverage_build_command end-to-end (both coverage jobs thread them through) — no .github change is needed. This is purely a per-repo build-setup task.