feat(coordination): event-sourced DecisionLog keystone (WP-0007 T5)

Append-only, totally-ordered-per-space decision log (in-process append authority;
git+lease later) with event types overlay/binding/alias/merge/fork, and a derived
fold to CoordinationState (aliases LWW, transitively-merged equivalence groups,
open overlays). derived = f(log); read-your-writes. 6 tests green. (blueprint §8.1)

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

tests/test_decision_log.py

@@ -0,0 +1,51 @@
+"""Tests for the event-sourced DecisionLog (SHARD-WP-0007 T5)."""
+
+from shard_wiki.coordination import DecisionLog, EventType
+
+
+def test_append_is_totally_ordered_per_space():
+    log = DecisionLog()
+    log.append("spaceA", EventType.ALIAS_SET, {"alias": "Home", "target": "shardA:Index"})
+    log.append("spaceA", EventType.ALIAS_SET, {"alias": "Start", "target": "shardA:Index"})
+    log.append("spaceB", EventType.ALIAS_SET, {"alias": "X", "target": "shardB:Y"})
+    seqs = [e.seq for e in log.events("spaceA")]
+    assert seqs == [0, 1]  # per-space monotonic
+    assert [e.seq for e in log.events("spaceB")] == [0]  # independent ordering
+
+
+def test_read_your_writes():
+    log = DecisionLog()
+    ev = log.append("s", EventType.ALIAS_SET, {"alias": "a", "target": "shardA:b"})
+    assert log.events("s")[-1] is ev
+
+
+def test_fold_reproduces_current_state():
+    log = DecisionLog()
+    log.append("s", EventType.ALIAS_SET, {"alias": "Home", "target": "shardA:Index"})
+    log.append("s", EventType.ALIAS_SET, {"alias": "Home", "target": "shardB:Main"})  # LWW
+    state = log.fold("s")
+    assert state.resolve_alias("Home") == "shardB:Main"
+    assert state.resolve_alias("missing") is None
+
+
+def test_fold_is_pure_function_of_log():
+    log = DecisionLog()
+    log.append("s", EventType.BINDING_MADE, {"members": ["shardA:Home", "shardB:Home"]})
+    first = log.fold("s")
+    second = log.fold("s")
+    assert first.equivalence_groups == second.equivalence_groups  # derived = f(log)
+
+
+def test_equivalence_groups_merge_transitively():
+    log = DecisionLog()
+    log.append("s", EventType.BINDING_MADE, {"members": ["a", "b"]})
+    log.append("s", EventType.BINDING_MADE, {"members": ["b", "c"]})
+    state = log.fold("s")
+    assert state.equivalent_to("a") == frozenset({"a", "b", "c"})
+    assert state.equivalent_to("lonely") == frozenset({"lonely"})
+
+
+def test_page_fork_creates_equivalence():
+    log = DecisionLog()
+    log.append("s", EventType.PAGE_FORKED, {"source": "shardA:Doc", "fork": "shardB:Doc"})
+    assert log.fold("s").equivalent_to("shardA:Doc") == frozenset({"shardA:Doc", "shardB:Doc"})