fix(docs): regenerate readme.svg; CI checks it is what its code draws

.github/workflows/ci.yml

@@ -59,6 +59,25 @@ jobs:
           go-version: '1.27.2'
       - run: go run golang.org/x/vuln/cmd/govulncheck@v1.8.0 ./...
 
+  readme-figure:
+    name: the README figure is what its code draws
+    # docs/readme.svg is generated by tools/compare/readme from this tree. v0.1.0
+    # shipped one drawn by an earlier state of the code (its x tick labels sat
+    # 1 px off what the tagged code draws); regenerating and comparing byte for
+    # byte keeps the README from showing anything the library does not do.
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v7
+      - uses: actions/setup-go@v7
+        with:
+          go-version: '1.27.2'
+      - name: Regenerate and compare
+        run: |
+          cd tools/compare
+          go run ./readme > /tmp/readme.svg
+          cmp /tmp/readme.svg ../../docs/readme.svg || {
+            echo "::error::docs/readme.svg is stale: run (cd tools/compare && go run ./readme > ../../docs/readme.svg)"; exit 1; }
+
   arch-native:
     name: arch ${{ matrix.label }}
     runs-on: ${{ matrix.os }}