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>

tests/test_write_path_integration.py

@@ -0,0 +1,58 @@
+"""End-to-end write-path test (SHARD-WP-0008 T6)."""
+
+from shard_wiki import InformationSpace
+from shard_wiki.adapters import FolderAdapter
+from shard_wiki.coordination import ApplyStatus
+from shard_wiki.provenance import OverlayState
+
+
+def _folder(tmp_path, name, files, writable=False):
+    root = tmp_path / name
+    root.mkdir(parents=True, exist_ok=True)
+    for rel, text in files.items():
+        (root / rel).write_text(text, encoding="utf-8")
+    return FolderAdapter(name, root, writable=writable)
+
+
+def test_edit_writethrough_on_writable_shard(tmp_path):
+    space = InformationSpace("team")
+    space.attach(_folder(tmp_path, "wikiW", {"Home.md": "old"}, writable=True))
+
+    result = space.edit("Home", "new content")
+    assert result.status is ApplyStatus.APPLIED
+    assert space.read("Home").body == "new content"  # persisted to the shard
+    assert space.read("Home").envelope.overlay_state is OverlayState.NONE  # overlay closed
+
+
+def test_edit_on_readonly_shard_keeps_local_draft(tmp_path):
+    space = InformationSpace("team")
+    space.attach(_folder(tmp_path, "wikiRO", {"Home.md": "canon"}))
+
+    result = space.edit("Home", "my local edit")
+    assert result.status is ApplyStatus.KEPT_DRAFT
+    # source untouched; union shows the draft as local truth, clearly flagged
+    page = space.read("Home")
+    assert page.body == "my local edit"
+    assert page.envelope.overlay_state is OverlayState.DRAFT
+
+
+def test_explicit_overlay_then_apply(tmp_path):
+    space = InformationSpace("team")
+    space.attach(_folder(tmp_path, "wikiW", {"Doc.md": "v1"}, writable=True))
+    ov = space.overlay("Doc", "v2")
+    assert space.read("Doc").envelope.overlay_state is OverlayState.DRAFT  # pending
+    result = space.apply_overlay(ov.overlay_id)
+    assert result.status is ApplyStatus.APPLIED
+    assert space.read("Doc").body == "v2"
+
+
+def test_stale_overlay_refuses_after_external_change(tmp_path):
+    space = InformationSpace("team")
+    shard = _folder(tmp_path, "wikiW", {"Doc.md": "v1"}, writable=True)
+    space.attach(shard)
+    ov = space.overlay("Doc", "from-v1")
+    # an external write moves the shard under the overlay
+    shard.write("Doc", "v1-prime")
+    result = space.apply_overlay(ov.overlay_id)
+    assert result.status is ApplyStatus.REFUSED_DRIFT
+    assert space.union.shard("wikiW").read("Doc").body == "v1-prime"  # not clobbered