feat(views): BackLinks derived view over the union link graph (WP-0010 T2)

For any page name, the set of pages that link to it: extract wikilinks from every
union page (new UnionGraph.iter_pages enumeration) and index the resolved ones by
target name. Red-links create no backlinks; entries carry source provenance; a
chorus target aggregates the backlinks of all members under one name. Derived/
recomputable, stores nothing canonical.

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

src/shard_wiki/views/__init__.py

@@ -6,6 +6,7 @@ stays out of core (L6) — these produce models, never rendered output. Per the
 package imports down (union/model/coordination/provenance) and is imported only by the orchestrator.
 """
 
+from shard_wiki.views.backlinks import BackLink, BackLinksIndex, build_backlinks
 from shard_wiki.views.links import (
     ResolvedLink,
     WikiLink,
@@ -18,4 +19,7 @@ __all__ = [
     "ResolvedLink",
     "extract_links",
     "resolve_links",
+    "BackLink",
+    "BackLinksIndex",
+    "build_backlinks",
 ]