feat(space): wire write path into InformationSpace; integration (WP-0008 T6)

edit()/overlay()/apply_overlay() on InformationSpace. edit() unifies the write
path through one principled route — draft overlay then apply: write-through-capable
target fast-forwards (APPLIED), read-only target keeps the draft as local truth
(KEPT_DRAFT), external drift refuses (no clobber). Integration tests cover all
four. 64 tests green, pyflakes clean. Flips WP-0008 done.

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

src/shard_wiki/space.py

@@ -9,7 +9,13 @@ a network API is a later workplan.
 from __future__ import annotations
 
 from shard_wiki.adapters import ShardAdapter, assert_conformant
-from shard_wiki.coordination import DecisionLog, EventType
+from shard_wiki.coordination import (
+    ApplyResult,
+    DecisionLog,
+    EventType,
+    Overlay,
+    OverlayEngine,
+)
 from shard_wiki.model import Page
 from shard_wiki.policy import DEFAULT_POLICY, Policy
 from shard_wiki.union import Resolution, UnionGraph
@@ -22,6 +28,7 @@ class InformationSpace:
         self.space_id = space_id
         self.log = DecisionLog()
         self.union = UnionGraph(space_id, log=self.log, policy=policy)
+        self.overlays = OverlayEngine(space_id, self.log)
 
     def attach(self, adapter: ShardAdapter) -> None:
         """Attach a shard — only if it passes conformance (verified profile, I-3/§6.6)."""
@@ -40,3 +47,25 @@ class InformationSpace:
     def read(self, name: str) -> Page:
         """Resolve and return the page (or the canonical pick of a chorus). KeyError if red-link."""
         return self.union.resolve(name).single()
+
+    def overlay(self, name: str, body: str, actor: str | None = None) -> Overlay:
+        """Draft a non-destructive overlay against the resolved page (overlay-before-mutation)."""
+        page = self.read(name)
+        return self.overlays.draft(page.identity, body, page.envelope.source_rev, actor=actor)
+
+    def apply_overlay(self, overlay_id: str) -> ApplyResult:
+        """Apply a draft overlay to its target shard (apply-under-drift, §8.6)."""
+        overlay = self.overlays.get(overlay_id)
+        if overlay is None:
+            raise KeyError(overlay_id)
+        adapter = self.union.shard(overlay.target.shard)
+        if adapter is None:
+            raise KeyError(overlay.target.shard)
+        return self.overlays.apply(overlay_id, adapter)
+
+    def edit(self, name: str, body: str, actor: str | None = None) -> ApplyResult:
+        """Edit a page through the one principled path: draft an overlay, then apply it. A
+        write-through-capable target fast-forwards (write-through); a read-only target keeps the
+        draft as local truth (I-5: overlay before mutation, always)."""
+        overlay = self.overlay(name, body, actor=actor)
+        return self.apply_overlay(overlay.overlay_id)

src/shard_wiki/union/resolver.py

@@ -65,7 +65,7 @@ class UnionGraph:
     def attach(self, adapter: ShardAdapter) -> None:
         self._shards.append(adapter)
 
-    def _shard(self, shard_id: str) -> ShardAdapter | None:
+    def shard(self, shard_id: str) -> ShardAdapter | None:
         return next((s for s in self._shards if s.shard_id == shard_id), None)
 
     def _read_all(self, key: str) -> list[Page]:
@@ -88,7 +88,7 @@ class UnionGraph:
         target = state.resolve_alias(name)
         if target is not None and ":" in target:
             shard_id, _, key = target.partition(":")
-            shard = self._shard(shard_id)
+            shard = self.shard(shard_id)
             if shard is not None:
                 try:
                     page = self._with_overlay(shard.read(key), overlays)
@@ -127,7 +127,7 @@ class UnionGraph:
         have = {p.identity for p in existing}
         out: list[Page] = []
         for identity, overlay in overlays.items():
-            if identity.key != name or identity in have or self._shard(identity.shard) is None:
+            if identity.key != name or identity in have or self.shard(identity.shard) is None:
                 continue
             env = ProvenanceEnvelope(
                 source_shard=identity.shard,