Parallel CI for many branches via workflow_dispatch and ci/* refs

If a GitHub Actions workflow triggers on push to one branch and has concurrency: group: <name>-${{ github.ref }} with cancel-in-progress, serial pushes cancel each other and every change waits a full build. Without editing the workflow, push each ready commit to its own ref (git push origin <sha>:refs/heads/ci/<name>) and start it with gh workflow run <file> --ref ci/<name> (needs a workflow_dispatch trigger). Distinct refs mean distinct concurrency groups, so they build in parallel. Also build a stacked ci/stack of all ready branches, and fast-forward the main branch to it once green. Pair this with one local incremental build slot that always takes the newest stack, so compile errors surface in minutes.