ci: run pytest on pull requests, with rsvg-convert available

docs.yml becomes ci.yml with a second job. The gate it adds is not really "run the
tests" — it is running the ones that have never run anywhere.

Every PNG assertion in the suite is gated on rsvg-convert, which no environment we
use has installed. That is 26 skips, all of them test_no_cream_on_transparent, the
invariant that stops a mark shipping stray white ink on every dark surface; plus 5
tests that hard-fail without it and 6 conditional blocks in test_assets.py that
quietly drop their PNG checks. So the test job installs librsvg2-bin and runs
rsvg-convert --version, because an apt-get that silently no-ops would leave the job
green with the whole census skipped.

Verified by CI rather than locally: this machine has no rsvg-convert, so the 31
affected tests cannot run here.

Author: lesnik512

.github/workflows/ci.yml

@@ -0,0 +1,34 @@
+name: CI
+
+on:
+  pull_request:
+
+jobs:
+  docs:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v6
+      - name: Install uv
+        uses: astral-sh/setup-uv@v7
+        with:
+          python-version: "3.11"
+      - name: Build site
+        run: uv run mkdocs build --strict
+
+  test:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v6
+      # librsvg2-bin supplies rsvg-convert. Without it every PNG assertion skips
+      # itself, so the version check is what keeps the job from passing empty.
+      - name: Install rsvg-convert
+        run: |
+          sudo apt-get update
+          sudo apt-get install -y librsvg2-bin
+          rsvg-convert --version
+      - name: Install uv
+        uses: astral-sh/setup-uv@v7
+        with:
+          python-version: "3.11"
+      - name: Run tests
+        run: uv run pytest