feat(incremental): I-2 digest + consistency-checker (WP-0011 T3)

A Merkle-style digest summarizes the derived tier (per-identity fingerprint +
incident edges as order-independent leaves) so equal states have equal digests
and the digest is stable under equivalent event orders. A ConsistencyChecker
recomputes the authoritative fold from the current source, compares it over a
sampled region, and on mismatch scoped-recomputes just the affected identities —
self-healing missed-delta drift, corrupted internal state, and vanished pages.
Makes derived = f(canonical) verified, not asserted.

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

src/shard_wiki/incremental/equivalence.py

@@ -136,6 +136,15 @@ class EquivalenceIndex:
 
     # -- queries -------------------------------------------------------------
 
+    def identities(self) -> frozenset[Identity]:
+        """All identities currently present in the index."""
+        return frozenset(self._entries)
+
+    def fingerprint(self, identity: Identity) -> str | None:
+        """The content fingerprint indexed for ``identity`` (None if absent) — a digest leaf."""
+        entry = self._entries.get(identity)
+        return entry.fingerprint if entry is not None else None
+
     def edges(self) -> frozenset[frozenset[Identity]]:
         """All equivalence edges (content + curator) among currently present identities."""
         present = self._entries.keys()