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>

src/shard_wiki/coordination/__init__.py

@@ -0,0 +1,10 @@
+"""coordination/ — the event-sourced decision log (L3, coordination-canonical state)."""
+
+from shard_wiki.coordination.decision_log import (
+    CoordinationState,
+    DecisionEvent,
+    DecisionLog,
+    EventType,
+)
+
+__all__ = ["DecisionLog", "DecisionEvent", "EventType", "CoordinationState"]

src/shard_wiki/coordination/decision_log.py

@@ -0,0 +1,124 @@
+"""The event-sourced coordination decision log — the keystone (CoreArchitectureBlueprint §8.1).
+
+Coordination-canonical state (overlays, equivalence bindings, aliases, merges, forks) is an
+**append-only decision log**, not a mutable file; the queryable *current* state is a **derived
+fold** of the log (tier-3 disposable). The log is **totally ordered per space** via a single
+**append authority** — here an in-process counter; a git-backed, lease-held authority is a later
+binding. That total order is what gives read-your-writes across readers (§8.6).
+
+`derived = f(canonical)`: :class:`CoordinationState` is always reproducible by replaying the log.
+"""
+
+from __future__ import annotations
+
+from collections.abc import Mapping
+from dataclasses import dataclass, field
+from datetime import datetime, timezone
+from enum import Enum
+from types import MappingProxyType
+from typing import Any
+
+__all__ = ["EventType", "DecisionEvent", "CoordinationState", "DecisionLog"]
+
+
+class EventType(Enum):
+    OVERLAY_CREATED = "overlay-created"
+    BINDING_MADE = "binding-made"
+    ALIAS_SET = "alias-set"
+    MERGE_DECIDED = "merge-decided"
+    PAGE_FORKED = "page-forked"
+
+
+@dataclass(frozen=True, slots=True)
+class DecisionEvent:
+    """One immutable, ordered decision. ``seq`` is the per-space total order."""
+
+    seq: int
+    space: str
+    type: EventType
+    payload: Mapping[str, Any]
+    actor: str | None = None
+    timestamp: datetime = field(default_factory=lambda: datetime.now(tz=timezone.utc))
+
+
+@dataclass(frozen=True, slots=True)
+class CoordinationState:
+    """The derived fold of a space's log: current aliases + equivalence groups + open overlays.
+
+    Disposable (tier-3): always recomputable from the log via :meth:`DecisionLog.fold`.
+    """
+
+    aliases: Mapping[str, str]
+    equivalence_groups: tuple[frozenset[str], ...]
+    open_overlays: Mapping[str, Mapping[str, Any]]
+
+    def resolve_alias(self, name: str) -> str | None:
+        return self.aliases.get(name)
+
+    def equivalent_to(self, identity: str) -> frozenset[str]:
+        """All identities equivalent to ``identity`` (including itself if bound), else just it."""
+        for group in self.equivalence_groups:
+            if identity in group:
+                return group
+        return frozenset({identity})
+
+
+class DecisionLog:
+    """In-memory append-only log, totally ordered per space (the append authority for a process).
+
+    A later binding swaps the storage for git + a per-space lease without changing this API.
+    """
+
+    def __init__(self) -> None:
+        self._events: dict[str, list[DecisionEvent]] = {}
+
+    def append(
+        self,
+        space: str,
+        type: EventType,
+        payload: Mapping[str, Any],
+        actor: str | None = None,
+    ) -> DecisionEvent:
+        seq = len(self._events.get(space, ()))
+        event = DecisionEvent(seq=seq, space=space, type=type, payload=dict(payload), actor=actor)
+        self._events.setdefault(space, []).append(event)
+        return event
+
+    def events(self, space: str) -> tuple[DecisionEvent, ...]:
+        """The space's events in append (total) order. Read-your-writes: a just-appended event
+        is present immediately."""
+        return tuple(self._events.get(space, ()))
+
+    def fold(self, space: str) -> CoordinationState:
+        """Replay the log into current coordination state (derived = f(log))."""
+        aliases: dict[str, str] = {}
+        overlays: dict[str, dict[str, Any]] = {}
+        groups: list[set[str]] = []
+
+        for event in self.events(space):
+            if event.type is EventType.ALIAS_SET:
+                aliases[event.payload["alias"]] = event.payload["target"]
+            elif event.type is EventType.BINDING_MADE:
+                _merge_group(groups, {str(m) for m in event.payload["members"]})
+            elif event.type is EventType.OVERLAY_CREATED:
+                overlays[event.payload["overlay_id"]] = dict(event.payload)
+            elif event.type is EventType.MERGE_DECIDED:
+                # A merge resolution may collapse an overlay; minimal handling for the slice.
+                overlays.pop(event.payload.get("overlay_id", ""), None)
+            elif event.type is EventType.PAGE_FORKED:
+                _merge_group(groups, {str(event.payload["source"]), str(event.payload["fork"])})
+
+        return CoordinationState(
+            aliases=MappingProxyType(dict(aliases)),
+            equivalence_groups=tuple(frozenset(g) for g in groups),
+            open_overlays=MappingProxyType({k: MappingProxyType(v) for k, v in overlays.items()}),
+        )
+
+
+def _merge_group(groups: list[set[str]], members: set[str]) -> None:
+    """Union-merge ``members`` into ``groups`` (any existing group sharing a member absorbs it)."""
+    touching = [g for g in groups if g & members]
+    for g in touching:
+        groups.remove(g)
+        members |= g
+    groups.append(members)