Add downgrade CI workflow with InterfaceI test group

This adds the standardized downgrade CI workflow following the SciMLBase.jl
template, configured to run the InterfaceI test group for lean testing
while verifying package compatibility with downgraded dependencies.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: claude

.github/workflows/Downgrade.yml

@@ -0,0 +1,36 @@
+name: Downgrade
+on:
+  pull_request:
+    branches:
+      - master
+    paths-ignore:
+      - 'docs/**'
+      - 'benchmark/**'
+  push:
+    branches:
+      - master
+    paths-ignore:
+      - 'docs/**'
+      - 'benchmark/**'
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        downgrade_mode: ['alldeps']
+        julia-version: ['1.10']
+        group: ['InterfaceI']
+    steps:
+      - uses: actions/checkout@v4
+      - uses: julia-actions/setup-julia@v2
+        with:
+          version: ${{ matrix.julia-version }}
+      - uses: julia-actions/julia-downgrade-compat@v2
+        with:
+          skip: Pkg,TOML
+      - uses: julia-actions/julia-buildpkg@v1
+      - uses: julia-actions/julia-runtest@v1
+        with:
+          ALLOW_RERESOLVE: false
+        env:
+          GROUP: ${{ matrix.group }}