Install uv without an action so the matrix jobs stop racing

act caches each action as one shared git clone under /root/.cache/act/<hash>
and re-pulls it per job. The three python matrix jobs start within the same
second on the self-hosted runner, race on that directory, and the loser dies
with "lstat /root/.cache/act/<hash>/.npmrc: no such file or directory" before
any test runs — a red main with zero suite output and a different victim each
push (3.12, then 3.11).

The action was only fetching a binary; the matrix interpreter is selected per
command by uvx --python. A run: step has no action-cache entry and cannot
race, and keeps the jobs parallel — serialising the matrix would cost 3x the
wall clock and still leave actions/checkout shared across four jobs. The uv
version is pinned under the same rule as ruff.

The accompanying test parses uses: directives rather than the raw text, so the
comment can still name the action it avoids, and uses re instead of PyYAML
because CI runs uvx pytest, whose environment holds pytest and nothing else.

.github/workflows/ci.yml

@@ -47,7 +47,29 @@ jobs:
       # download path setup-python relies on does not work on the self-hosted Gitea
       # runner (all three matrix jobs failed at setup there while passing on GitHub);
       # uv works identically on both.
-      - uses: astral-sh/setup-uv@fac544c07dec837d0ccb6301d7b5580bf5edae39 # v8.2.0
+      #
+      # Installed by a plain `run:` step rather than astral-sh/setup-uv, because
+      # act -- the engine behind the Gitea runner -- caches each ACTION as a single
+      # shared git clone under /root/.cache/act/<hash> and re-pulls it per job. The
+      # three matrix jobs start within the same second on one runner, so they race
+      # on that directory and whichever loses dies with
+      #
+      #   lstat /root/.cache/act/<hash>/.npmrc: no such file or directory
+      #
+      # before a single test runs -- a red `main` with zero suite output, and a
+      # different victim each time (3.12 on one push, 3.11 on the next). A `run:`
+      # step has no action-cache entry, so it cannot race. Serialising the matrix
+      # would have been the other option; it costs 3x the wall clock and still
+      # leaves actions/checkout sharing a cache across the four jobs.
+      #
+      # Pinned for the same reason ruff is pinned below: an unpinned uv means any
+      # upstream release can turn main red with no code change here.
+      - name: install uv
+        env:
+          UV_VERSION: "0.11.21"
+        run: |
+          curl -LsSf "https://astral.sh/uv/${UV_VERSION}/install.sh" | sh
+          echo "$HOME/.local/bin" >> "$GITHUB_PATH"
 
       - name: ruff
         # Pinned: an unpinned ruff means any upstream release can turn main red