feat(cms): U3 derive article dates from content/ front-matter

Replace the hand-maintained ARTICLE_DATES registry as the source of
truth with dates read from content/ front-matter, so publishing a post
via the CMS needs no manual registry edit (plan R4).

- tools/content_index.py: load_dates() reads content/ front-matter into
  {source_folder/slug: {published|modified: iso}}. Standalone (os+yaml),
  no import cycle with seo_inject.
- tools/seo_inject.py: ARTICLE_DATES is now {**static_fallback,
  **content_dates}; content wins. The static table is kept only as a
  fallback for entries not present in content/.
- migration now preserves the published-vs-modified distinction
  (date_type), so the switch is byte-identical: content-derived dates
  equal the old static table exactly (17/17, incl. the one 'modified').

Verified: full `seo_inject --all --sitemap` + build_listings +
build_botlinks run produces ZERO changes to any site page. build_content
--check still reproduces all 51 pages.

build_botlinks.py needs no change (it scrapes built pages, which are a
pure function of content/). Listings get content-driven dates for free
via seo_inject.ARTICLE_DATES.

tools/tests/test_content_index.py: 5 tests (dates == legacy registry,
merged registry unchanged, published/modified distinction, events key on
projects/<slug>, one ISO date per entry).

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

tools/migrate_pages_to_content.py

@@ -72,19 +72,23 @@ def _text(node):
 
 
 def _iso_date(source_folder, slug, en_html=None):
-    """ISO date: prefer the central ARTICLE_DATES registry, then fall back to
-    reading it off the English page (mirrors build_listings' behaviour)."""
+    """(iso, kind) for an entry: prefer the central ARTICLE_DATES registry
+    (preserving whether the date is a 'published' or 'modified' one), then fall
+    back to reading it off the English page (mirrors build_listings)."""
     key = f"{source_folder}/{slug}"
     rec = seo_inject.ARTICLE_DATES.get(key, {})
-    iso = rec.get("published") or rec.get("modified")
-    if iso:
-        return iso
+    if rec.get("published"):
+        return rec["published"], "published"
+    if rec.get("modified"):
+        return rec["modified"], "modified"
     if en_html is not None:
         try:
-            return seo_inject.extract_publish_date(en_html)
+            iso = seo_inject.extract_publish_date(en_html)
+            if iso:
+                return iso, "published"
         except Exception:
-            return None
-    return None
+            pass
+    return None, None
 
 
 def _parse_hero(figure):
@@ -154,10 +158,13 @@ def parse_page(html, source_folder, slug, lang, en_html=None):
         if cat and not re.search(r"\bpublished\b|\d{4}", cat, re.I):
             record["category"] = cat
 
-    iso = _iso_date(source_folder, slug, en_html=en_html)
+    iso, kind = _iso_date(source_folder, slug, en_html=en_html)
     if not iso:
         return None, "no date (missing from ARTICLE_DATES and unparseable)"
     record["date"] = iso
+    if kind == "modified":
+        # 'published' is the implicit default; only record the exception.
+        record["date_type"] = "modified"
     if date_line:
         record["date_line"] = date_line
 
@@ -175,7 +182,7 @@ def parse_page(html, source_folder, slug, lang, en_html=None):
 # Deterministic front-matter key order for readable, stable output.
 _KEY_ORDER = [
     "type", "slug", "lang", "source_folder",
-    "title", "subtitle", "category", "date", "date_line", "hero", "body_format",
+    "title", "subtitle", "category", "date", "date_type", "date_line", "hero", "body_format",
 ]