ci: split downgrade testing into its own workflow (#123)

Move the conditional downgrade matrix leg out of CI.yml into a
separate Downgrade.yml, running only against Julia lts with mode:
deps, matching the rest of the fleet. CI.yml goes back to a plain
version matrix.

Co-authored-by: Claude Sonnet 5 <noreply@anthropic.com>

Author: sethaxen

.github/workflows/CI.yml

@@ -16,7 +16,7 @@ concurrency:
 
 jobs:
   test:
-    name: ${{ matrix.downgrade && 'Downgrade / ' || '' }}Julia ${{ matrix.version }} - ${{ matrix.os }}
+    name: Julia ${{ matrix.version }} - ${{ matrix.os }}
     runs-on: ${{ matrix.os }}
     strategy:
       fail-fast: false
@@ -28,30 +28,17 @@ jobs:
           - 'pre'
         os:
           - ubuntu-latest
-        downgrade: [false]
         include:
           - version: '1'
             os: windows-latest
-            downgrade: false
           - version: '1'
             os: macOS-latest
-            downgrade: false
-          - version: 'min'
-            os: ubuntu-latest
-            downgrade: true
     steps:
       - uses: actions/checkout@v7
       - uses: julia-actions/setup-julia@v3
-        id: setup-julia
         with:
           version: ${{ matrix.version }}
       - uses: julia-actions/cache@v3
-      - uses: julia-actions/julia-downgrade-compat@v2
-        with:
-          julia_version: ${{ steps.setup-julia.outputs.julia-version }}
-          skip: Dates,Random
-        if: ${{ matrix.downgrade }}
-        name: Downgrade dependencies to oldest supported versions
       - uses: julia-actions/julia-buildpkg@v1
       - uses: julia-actions/julia-runtest@v1
       - uses: julia-actions/julia-processcoverage@v1

.github/workflows/Downgrade.yml

@@ -0,0 +1,39 @@
+name: Downgrade
+on:
+  pull_request:
+  push:
+    branches:
+      - main
+  workflow_dispatch:
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: ${{ startsWith(github.ref, 'refs/pull/') }}
+
+jobs:
+  test:
+    name: Downgrade / Julia ${{ matrix.version }} - ${{ matrix.os }}
+    runs-on: ${{ matrix.os }}
+    strategy:
+      fail-fast: false
+      matrix:
+        version: ['lts']
+        os: [ubuntu-latest]
+    steps:
+      - uses: actions/checkout@v7
+      - uses: julia-actions/setup-julia@v3
+        with:
+          version: ${{ matrix.version }}
+      - uses: julia-actions/cache@v3
+      - uses: julia-actions/julia-downgrade-compat@v2
+        with:
+          mode: deps
+      - uses: julia-actions/julia-buildpkg@v1
+      - uses: julia-actions/julia-runtest@v1
+        with:
+          allow_reresolve: false
+          force_latest_compatible_version: false
+      - uses: julia-actions/julia-processcoverage@v1
+      - uses: codecov/codecov-action@v7
+        with:
+          files: lcov.info