Report test coverage in the CI run summary

[jstayton]

npm test runs node --test with no coverage flags, so seeing coverage meant passing --experimental-test-coverage by hand. This adds a separate Test coverage job that posts the report to the run summary and uploads coverage/lcov.info as a coverage artifact — the same shape as the job in vision-download: https://github.com/TRUEPIC/vision-download/actions/runs/33080309557

No thresholds. Coverage is reported, not enforced.

It uses Node's built-in --experimental-test-coverage with the lcov reporter rather than nyc or c8, so nothing new is added to the dependency tree. The job runs once on 26.x instead of across the 22/24/26 matrix, so the run gets one summary rather than three; the existing Lint & test job is untouched.

Two things in the diff that look incidental but aren't:

set -o pipefail in the reporting step — npm run test:coverage | tee returns tee's exit status, not npm's. I checked with a deliberately failing test: exit 0 without it, exit 1 with it. Without that line the coverage job reports green on a broken suite.

pretest:coverage — the lcov reporter won't create its destination directory, it exits 7 on a missing coverage/. The pre-hook does the mkdirSync rather than a shell mkdir -p, so it stays portable.

Rendered summary looks like this:

----------------------------------------------------------
file      | line % | branch % | funcs % | uncovered lines
----------------------------------------------------------
src       |        |          |         |
 error.js | 100.00 |   100.00 |  100.00 |
 main.js  | 100.00 |   100.00 |  100.00 |
----------------------------------------------------------
all files | 100.00 |   100.00 |  100.00 |
----------------------------------------------------------

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