Vendor shared SEO rule engine + CI drift guard (no pipeline wiring yet)

src/seo/rules.py is a byte-for-byte copy of the canonical seo_rules.py
(git.chemavx.xyz/chemavx/chemavx-seo-tools). CI step "Verify vendored SEO engine"
clones the canonical and fails the build on any divergence; `make sync-seo` /
`make check-seo-sync` cover the local-canonical workflow. Nothing imports this in
a runtime path yet — generate_seo_fields integration comes next behind SEO_AUTOFILL.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/seo/__init__.py

@@ -0,0 +1,2 @@
+"""Vendored SEO rule engine (see rules.py). Re-export check_post/score for the bot."""
+from .rules import check_post, score, internal_links, Violation  # noqa: F401