ci: run browser checks in parallel and cancel superseded PR runs

[rosek86]

Run Chromium, WebKit, and Firefox verification in parallel and cancel superseded runs of the same PR. The previous successful check job took 5m55s; its browser suites ran sequentially.

Both CI and npm release verification now call a shared workflow. It builds and packs once, then sends that exact build to three browser jobs and an independent consumer-verification job. Each job checks out the commit recorded by the build. Browser failures retain separate diagnostic artifacts and do not cancel the other browser jobs.

Only after every verification succeeds does the workflow generate the release manifest and upload the verified package and optional Pages artifact. Preserve the top-level check gate, Pages deployment conditions, npm environment, and publication checks. Main and release workflows retain non-cancelling concurrency. Local prepare:release remains the full sequential verification command.

Validation:

• npm run lint:fix, npm run format, and npm run check passed.
• npm run check:package passed for the packed archive.
• Actionlint passed for the reusable workflow and callers.
• Executed the actual manifest step against a packed fixture; verified integrity/commit metadata and rejection of a mismatched commit.
• Exercised the final guard: success passes; failure, skipped, and cancelled browser results fail.
• Final GitHub Actions run passed: https://github.com/rosek86/smithkit/actions/runs/38509110775
• Confirmed automatic PR cancellation: the next push cancelled run 38509076432 without a manual cancellation request.
• Confirmed all three browser jobs and consumer verification run concurrently, with at most four runners active.
• Measured from build-job start to the top-level check completion: 4m08s versus 5m55s for the previous sequential check (1m47s / 30% shorter in this comparison; runner and network variance still applies).