ci: add centralized sublibrary downgrade CI

Adds DowngradeSublibraries.yml calling the reusable
SciML/.github sublibrary-downgrade.yml@v1 workflow to run downgrade
tests for each lib/* sublibrary package.

Co-Authored-By: Chris Rackauckas <accounts@chrisrackauckas.com>
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: ChrisRackauckas

.github/workflows/DowngradeSublibraries.yml

@@ -0,0 +1,23 @@
+name: Downgrade Sublibraries
+on:
+  pull_request:
+    branches:
+      - master
+    paths-ignore:
+      - 'docs/**'
+  push:
+    branches:
+      - master
+    paths-ignore:
+      - 'docs/**'
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: ${{ github.ref != 'refs/heads/master' }}
+jobs:
+  downgrade-sublibraries:
+    uses: "SciML/.github/.github/workflows/sublibrary-downgrade.yml@v1"
+    secrets: "inherit"
+    with:
+      julia-version: "lts"
+      skip: "RecursiveArrayTools, Pkg, TOML, Statistics, LinearAlgebra, SparseArrays, InteractiveUtils, Random, Test"
+      allow-reresolve: true