Run CI checks as parallel jobs

.github/workflows/ci.yml

@@ -16,18 +16,49 @@ concurrency:
   cancel-in-progress: ${{ github.event_name == 'pull_request' }}
 
 jobs:
-  checks:
+  # The four jobs below were one serial `checks` job. Splitting them lets
+  # GitHub run them concurrently, so a PR's wall-clock is the slowest job
+  # rather than the sum of all of them, and a lint or structural failure
+  # reports in about a minute instead of waiting behind the build.
+  lint:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4
       - uses: oven-sh/setup-bun@v2
         with:
           bun-version: latest
       - run: bun install --frozen-lockfile
-      - run: bun run typecheck
       - run: bun run lint
+
+  typecheck:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: oven-sh/setup-bun@v2
+        with:
+          bun-version: latest
+      - run: bun install --frozen-lockfile
+      - run: bun run typecheck
+
+  build-test:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: oven-sh/setup-bun@v2
+        with:
+          bun-version: latest
+      - run: bun install --frozen-lockfile
       - run: bun run build
       - run: bun run test
+
+  structural:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: oven-sh/setup-bun@v2
+        with:
+          bun-version: latest
+      - run: bun install --frozen-lockfile
       - name: Structural check self-tests
         run: bun test scripts/checks/test
       - run: bun run check:deletion