feat(coordination): apply-under-drift (WP-0008 T4)

OverlayEngine.apply: read-only target → KEPT_DRAFT; base_rev==current →
fast-forward write-through (APPLIED, MERGE_DECIDED closes overlay); drift →
REFUSED_DRIFT (no clobber, I-5). 5 tests green, pyflakes clean. (blueprint §8.6)

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

tests/test_apply.py

@@ -0,0 +1,66 @@
+"""Tests for apply-under-drift (SHARD-WP-0008 T4)."""
+
+from shard_wiki.adapters import FolderAdapter
+from shard_wiki.coordination import ApplyStatus, DecisionLog, OverlayEngine
+from shard_wiki.model import Identity
+
+
+def _writable(tmp_path, files):
+    for rel, text in files.items():
+        (tmp_path / rel).write_text(text, encoding="utf-8")
+    return FolderAdapter("w", tmp_path, writable=True)
+
+
+def test_fast_forward_apply_writes_through(tmp_path):
+    shard = _writable(tmp_path, {"Home.md": "old"})
+    eng = OverlayEngine("space", DecisionLog())
+    base = shard.current_rev("Home")
+    ov = eng.draft(Identity("w", "Home"), "new", base_rev=base)
+
+    result = eng.apply(ov.overlay_id, shard)
+    assert result.status is ApplyStatus.APPLIED
+    assert shard.read("Home").body == "new"  # written through
+    assert eng.get(ov.overlay_id) is None  # overlay closed in the fold
+
+
+def test_drift_refuses_without_clobber(tmp_path):
+    shard = _writable(tmp_path, {"Home.md": "old"})
+    eng = OverlayEngine("space", DecisionLog())
+    ov = eng.draft(Identity("w", "Home"), "mine", base_rev="STALE-REV")
+
+    result = eng.apply(ov.overlay_id, shard)
+    assert result.status is ApplyStatus.REFUSED_DRIFT
+    assert shard.read("Home").body == "old"  # not clobbered (I-5)
+    assert eng.get(ov.overlay_id) is not None  # overlay still open
+
+
+def test_read_only_target_keeps_draft(tmp_path):
+    (tmp_path / "Home.md").write_text("canon", encoding="utf-8")
+    ro = FolderAdapter("ro", tmp_path)  # not writable
+    eng = OverlayEngine("space", DecisionLog())
+    ov = eng.draft(Identity("ro", "Home"), "my edit", base_rev=ro.current_rev("Home"))
+
+    result = eng.apply(ov.overlay_id, ro)
+    assert result.status is ApplyStatus.KEPT_DRAFT
+    assert ro.read("Home").body == "canon"  # source untouched
+    assert eng.get(ov.overlay_id) is not None  # local truth retained
+
+
+def test_new_page_fast_forwards(tmp_path):
+    shard = _writable(tmp_path, {})
+    eng = OverlayEngine("space", DecisionLog())
+    ov = eng.draft(Identity("w", "Fresh"), "brand new", base_rev=None)  # didn't exist
+    result = eng.apply(ov.overlay_id, shard)
+    assert result.status is ApplyStatus.APPLIED
+    assert shard.read("Fresh").body == "brand new"
+
+
+def test_wrong_adapter_is_rejected(tmp_path):
+    shard = _writable(tmp_path, {"Home.md": "x"})
+    eng = OverlayEngine("space", DecisionLog())
+    ov = eng.draft(Identity("other", "Home"), "y", base_rev=None)
+    try:
+        eng.apply(ov.overlay_id, shard)
+        raise AssertionError("expected ValueError")
+    except ValueError:
+        pass