feat(coordination): patch rendering (unified diff) (WP-0008 T3)

render_patch(overlay, base_body) → Patch (pure difflib unified diff, target-
labelled); is_empty when unchanged. 3 tests green, pyflakes clean. (ADR-05)

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

src/shard_wiki/coordination/__init__.py

@@ -7,6 +7,7 @@ from shard_wiki.coordination.decision_log import (
     EventType,
 )
 from shard_wiki.coordination.overlay import Overlay, OverlayEngine
+from shard_wiki.coordination.patch import Patch, render_patch
 
 __all__ = [
     "DecisionLog",
@@ -15,4 +16,6 @@ __all__ = [
     "CoordinationState",
     "Overlay",
     "OverlayEngine",
+    "Patch",
+    "render_patch",
 ]

src/shard_wiki/coordination/patch.py

@@ -0,0 +1,38 @@
+"""Patch rendering — an overlay as a reviewable diff (FederationRequirements ADR-05).
+
+A pure function over (base body, overlay body): the auditable change proposal that an overlay
+becomes before it is applied. Markdown/native text in this slice (lossless); a lossy
+native-syntax-with-fidelity-report variant is later (TSD §A.6).
+"""
+
+from __future__ import annotations
+
+import difflib
+from dataclasses import dataclass
+
+from shard_wiki.coordination.overlay import Overlay
+from shard_wiki.model import Identity
+
+__all__ = ["Patch", "render_patch"]
+
+
+@dataclass(frozen=True, slots=True)
+class Patch:
+    target: Identity
+    diff: str
+
+    @property
+    def is_empty(self) -> bool:
+        return self.diff == ""
+
+
+def render_patch(overlay: Overlay, base_body: str) -> Patch:
+    """Render ``base_body`` → ``overlay.body`` as a unified diff against the overlay target."""
+    label = str(overlay.target)
+    lines = difflib.unified_diff(
+        base_body.splitlines(keepends=True),
+        overlay.body.splitlines(keepends=True),
+        fromfile=f"a/{label}",
+        tofile=f"b/{label}",
+    )
+    return Patch(target=overlay.target, diff="".join(lines))