feat(union): overlay-aware resolution (WP-0008 T5)

resolve() layers open overlays onto canonical pages (overlay_state=DRAFT always
surfaced; overlaid body projected when policy.show_drafts); draft-only edits make
a not-yet-existing page resolvable. Never hides an unapplied overlay (I-4). Policy
gains show_drafts. 4 tests green, pyflakes clean.

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

src/shard_wiki/policy/__init__.py

@@ -31,6 +31,9 @@ class Policy:
 
     canonical_source: CanonicalSource = CanonicalSource.CHORUS
     designated_shard: str | None = None
+    # Whether an unapplied overlay's body is projected over its canonical page on read. Either
+    # way the overlay is never *hidden* — overlay_state is always surfaced in provenance.
+    show_drafts: bool = True
 
 
 DEFAULT_POLICY = Policy()