feat(policy,union): policy leaf + UnionGraph resolution with chorus (WP-0007 T6)

policy/ leaf (CanonicalSource presets, default chorus). union/ UnionGraph:
identity-keyed resolve (alias-redirect via log fold → union lookup → chorus →
red-link); chorus records divergent peers in each page's provenance envelope
(union without erasure); designated-canonical orders the pick. Imports down only.
6 tests green. (blueprint §8.4, ADR-01)

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

src/shard_wiki/policy/__init__.py

@@ -0,0 +1,36 @@
+"""policy/ — the configurable policy surface, a dependency-free leaf (blueprint §10, §11).
+
+Mechanism over policy (I-7): core mechanisms read policy *choices* from here; they never
+hard-code one. This leaf holds only the presets + a pure ``resolve``-style selector. Mechanism
+(how a choice is honoured) lives in ``coordination``/``union``/``projection``, never here.
+"""
+
+from __future__ import annotations
+
+from dataclasses import dataclass
+from enum import Enum
+
+__all__ = ["CanonicalSource", "Policy", "DEFAULT_POLICY"]
+
+
+class CanonicalSource(Enum):
+    """Resolution policy over a divergent/equivalent set (FederationArchitecture T9). Detection
+    is core; this is only the *resolution* choice."""
+
+    CHORUS = "chorus"  # default: present all versions, none privileged
+    DESIGNATED_CANONICAL = "designated-canonical"
+    GIT_MERGE = "git-merge"
+    VOTE = "vote"
+    OVERLAY_ONLY = "overlay-only"
+
+
+@dataclass(frozen=True, slots=True)
+class Policy:
+    """A space's policy choices. Extended as the policy surface grows (freshness, compaction,
+    execution, tenant mapping — blueprint §10); the slice needs canonical-source + designation."""
+
+    canonical_source: CanonicalSource = CanonicalSource.CHORUS
+    designated_shard: str | None = None
+
+
+DEFAULT_POLICY = Policy()