ci: add manual downstream workflow for recipe matrix

Introduce a workflow_dispatch-only workflow that runs each downstream TOML
recipe (conda, datasette, devpi, hatch, pytest, python-lsp-server, tox) on
ubuntu-latest with Python 3.12 and uv. Matrix jobs are independent
(fail-fast: false) and time out after 120 minutes. Trigger from Actions and,
when validating a change that needs it, select the relevant branch under
"Use workflow from" (for example a pull request head branch).

Co-authored-by: Cursor AI <ai@cursor.sh>
Co-authored-by: Composer <composer@cursor.com>

Author: 3 people

.github/workflows/downstream.yml

@@ -0,0 +1,36 @@
+# Downstream checks are manual-only. For a PR: Actions → downstream → Run workflow,
+# then pick the PR branch under "Use workflow from".
+name: downstream
+
+on:
+  workflow_dispatch:
+
+permissions: {}
+
+jobs:
+  recipe:
+    name: downstream (${{ matrix.recipe }})
+    runs-on: ubuntu-latest
+    timeout-minutes: 120
+    strategy:
+      fail-fast: false
+      matrix:
+        recipe:
+          - conda
+          - datasette
+          - devpi
+          - hatch
+          - pytest
+          - python-lsp-server
+          - tox
+    steps:
+      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+        with:
+          persist-credentials: false
+      - uses: actions/setup-python@a309ff8b426b58ec0e2a45f0f869d46889d02405 # v6.2.0
+        with:
+          python-version: "3.12"
+          allow-prereleases: true
+      - uses: astral-sh/setup-uv@08807647e7069bb48b6ef5acd8ec9567f424441b # v8.1.0
+      - name: Run downstream recipe
+        run: uv run downstream/run_downstream.py "${{ matrix.recipe }}"