blogimport: published-posts source contract + blog-side draft (76/76)

source.sx refactored to a single published-posts batch query returning full rows
(incl. lexical) — the existing post-by-id/slug DTO lacks lexical (sx_content/html
only), so the canonical lexical->blocks path needs a dedicated migration provider.
backfill-ids! now filters client-side (no extra query).

drafts/published-posts.sx + drafts/README.md: paste-ready blog-app change (defquery +
SqlBlogService.list_published_posts returning rows incl. raw lexical). README updated.
source 21/21; total 76/76.

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

lib/blogimport/scoreboard.json

@@ -3,9 +3,9 @@
     "lexical": {"pass": 23, "fail": 0},
     "import": {"pass": 21, "fail": 0},
     "verify": {"pass": 11, "fail": 0},
-    "source": {"pass": 20, "fail": 0}
+    "source": {"pass": 21, "fail": 0}
   },
-  "total_pass": 75,
+  "total_pass": 76,
   "total_fail": 0,
-  "total": 75
+  "total": 76
 }