ci: re-enable Downgrade job (strict, runs naturally) (#50)

Remove `if: false` so the downgrade job actually runs instead of being
skipped. Kept strict (no allow-reresolve, julia 1.10, floors unchanged).
Expected to be RED until the modern Lux/AD stack resolves (test reworked
in #49) — this is a natural failure, not a disabled job.

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

Author: 3 people

.github/workflows/Downgrade.yml

@@ -12,7 +12,7 @@ on:
       - 'docs/**'
 jobs:
   downgrade:
-    if: false
+    # Runs strict (allow_reresolve=false); expected RED until modern Lux/AD stack resolves (test reworked in #49)
     name: "Downgrade"
     uses: "SciML/.github/.github/workflows/downgrade.yml@v1"
     with: