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>

tests/test_overlay.py

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+"""Tests for the overlay model + draft (SHARD-WP-0008 T2)."""
+
+from shard_wiki.coordination import DecisionLog, EventType, OverlayEngine
+from shard_wiki.model import Identity
+from shard_wiki.provenance import OverlayState
+
+
+def test_draft_records_event_and_is_retrievable():
+    log = DecisionLog()
+    eng = OverlayEngine("space", log)
+    ov = eng.draft(Identity("shardA", "Home"), "new body", base_rev="r1")
+
+    assert ov.state is OverlayState.DRAFT
+    assert ov.body == "new body"
+    # recorded in the log (coordination-canonical)
+    events = log.events("space")
+    assert len(events) == 1 and events[0].type is EventType.OVERLAY_CREATED
+    # retrievable through the fold by its id
+    got = eng.get(ov.overlay_id)
+    assert got is not None
+    assert got.target == Identity("shardA", "Home")
+    assert got.base_rev == "r1"
+    assert got.body == "new body"
+
+
+def test_open_overlays_lists_drafts():
+    log = DecisionLog()
+    eng = OverlayEngine("space", log)
+    eng.draft(Identity("s", "A"), "a", base_rev=None)
+    eng.draft(Identity("s", "B"), "b", base_rev=None)
+    assert {o.target.key for o in eng.open_overlays()} == {"A", "B"}
+
+
+def test_unknown_overlay_is_none():
+    assert OverlayEngine("space", DecisionLog()).get("nope") is None
+
+
+def test_overlay_payload_round_trips():
+    log = DecisionLog()
+    eng = OverlayEngine("space", log)
+    ov = eng.draft(Identity("shardA", "sub/Page"), "x", base_rev="r9")
+    assert eng.get(ov.overlay_id) == ov  # payload reconstructs an equal Overlay