feat(coordination): Overlay model + OverlayEngine.draft (WP-0008 T2)

Overlay value type (id, target, base_rev, body, state) recorded as an
OVERLAY_CREATED decision-log event (coordination-canonical); get()/open_overlays()
reconstruct from the fold. 4 tests green, pyflakes clean. (ADR-05, blueprint §8.2)

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

src/shard_wiki/coordination/__init__.py

@@ -6,5 +6,13 @@ from shard_wiki.coordination.decision_log import (
     DecisionLog,
     EventType,
 )
+from shard_wiki.coordination.overlay import Overlay, OverlayEngine
 
-__all__ = ["DecisionLog", "DecisionEvent", "EventType", "CoordinationState"]
+__all__ = [
+    "DecisionLog",
+    "DecisionEvent",
+    "EventType",
+    "CoordinationState",
+    "Overlay",
+    "OverlayEngine",
+]

src/shard_wiki/coordination/overlay.py

@@ -0,0 +1,77 @@
+"""Overlay engine — overlay-before-mutation (FederationRequirements ADR-05, blueprint §8.2).
+
+An overlay is a non-destructive local edit against a page. It is **coordination-canonical**: an
+``OVERLAY_CREATED`` event in the decision log (§8.1), not a mutable side file. The current set
+of open overlays is the log fold. ``draft`` records one; ``apply`` (T4) resolves it under drift.
+"""
+
+from __future__ import annotations
+
+import uuid
+from collections.abc import Mapping
+from dataclasses import dataclass
+from typing import Any
+
+from shard_wiki.coordination.decision_log import DecisionLog, EventType
+from shard_wiki.model import Identity
+from shard_wiki.provenance import OverlayState
+
+__all__ = ["Overlay", "OverlayEngine"]
+
+
+@dataclass(frozen=True, slots=True)
+class Overlay:
+    """A non-destructive edit: the proposed ``body`` for ``target``, authored against
+    ``base_rev`` (the shard revision seen at draft time, for drift detection)."""
+
+    overlay_id: str
+    target: Identity
+    base_rev: str | None
+    body: str
+    state: OverlayState = OverlayState.DRAFT
+
+    def to_payload(self) -> dict[str, Any]:
+        return {
+            "overlay_id": self.overlay_id,
+            "target_shard": self.target.shard,
+            "target_key": self.target.key,
+            "base_rev": self.base_rev,
+            "body": self.body,
+            "state": self.state.value,
+        }
+
+    @classmethod
+    def from_payload(cls, payload: Mapping[str, Any]) -> Overlay:
+        return cls(
+            overlay_id=payload["overlay_id"],
+            target=Identity(payload["target_shard"], payload["target_key"]),
+            base_rev=payload["base_rev"],
+            body=payload["body"],
+            state=OverlayState(payload.get("state", OverlayState.DRAFT.value)),
+        )
+
+
+class OverlayEngine:
+    def __init__(self, space: str, log: DecisionLog) -> None:
+        self.space = space
+        self.log = log
+
+    def draft(
+        self,
+        target: Identity,
+        body: str,
+        base_rev: str | None,
+        actor: str | None = None,
+    ) -> Overlay:
+        """Create a draft overlay and record it in the decision log (coordination-canonical)."""
+        overlay = Overlay(uuid.uuid4().hex, target, base_rev, body)
+        self.log.append(self.space, EventType.OVERLAY_CREATED, overlay.to_payload(), actor=actor)
+        return overlay
+
+    def get(self, overlay_id: str) -> Overlay | None:
+        payload = self.log.fold(self.space).open_overlays.get(overlay_id)
+        return Overlay.from_payload(payload) if payload is not None else None
+
+    def open_overlays(self) -> tuple[Overlay, ...]:
+        state = self.log.fold(self.space)
+        return tuple(Overlay.from_payload(p) for p in state.open_overlays.values())