fix: lower the pipeline floor to Python 3.9

The 3.12 guard rested on generate_readme f-strings that have since been
rewritten. Every script compiles under CPython 3.9.25, 3.10 and 3.11, and
only generate_pack's parenthesized context managers fail under 3.8. Under
3.9 the offline pipeline (--skip-packs) completed with no change to the
committed artifacts, the 15 manifests matched 3.12 byte for byte, and the
RomM pack was identical. CI now compiles every script under a real 3.9.

.github/workflows/validate.yml

@@ -165,12 +165,14 @@ jobs:
       - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1  # v7
 
       # install.py stays on Python 3.8: the suite runs it end to end under a
-      # real 3.8 interpreter, which import-only and grammar checks miss. The
-      # last version listed is the default python.
+      # real 3.8 interpreter, which import-only and grammar checks miss. 3.9
+      # is the floor pipeline.py states for the build scripts, compiled under
+      # a real 3.9. The last version listed is the default python.
       - uses: actions/setup-python@5fda3b95a4ea91299a34e894583c3862153e4b97  # v7
         with:
           python-version: |
             3.8
+            3.9
             3.12
 
       # markdown: the site validation test renders a wiki line to prove the