feat(adapters): writable FolderAdapter + positive write conformance (WP-0008 T1)

FolderAdapter(writable=True) declares WRITE+PER_PAGE, implements write() and
current_rev() (mtime token for drift detection). Conformance gains a
content-preserving positive write probe for WRITE-claiming adapters. 5 tests
green, full suite green, pyflakes clean.

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

tests/test_writable_adapter.py

@@ -0,0 +1,41 @@
+"""Tests for the writable FolderAdapter + positive write conformance (SHARD-WP-0008 T1)."""
+
+import pytest
+
+from shard_wiki.adapters import FolderAdapter, assert_conformant, run_conformance
+from shard_wiki.model import NotSupported, Verb
+
+
+def test_writable_round_trip(tmp_path):
+    shard = FolderAdapter("w", tmp_path, writable=True)
+    rev_before = shard.current_rev("New")
+    assert rev_before is None
+    page = shard.write("New", "hello")
+    assert page.body == "hello"
+    assert shard.read("New").body == "hello"
+    assert shard.current_rev("New") is not None
+
+
+def test_writable_profile_supports_write(tmp_path):
+    prof = FolderAdapter("w", tmp_path, writable=True).profile()
+    assert prof.supports(Verb.WRITE)
+
+
+def test_read_only_still_rejects_write(tmp_path):
+    (tmp_path / "Home.md").write_text("x", encoding="utf-8")
+    with pytest.raises(NotSupported):
+        FolderAdapter("ro", tmp_path).write("Home", "y")
+
+
+def test_conformance_passes_for_writable(tmp_path):
+    (tmp_path / "Home.md").write_text("body", encoding="utf-8")
+    report = assert_conformant(FolderAdapter("w", tmp_path, writable=True))
+    assert report.ok
+    assert any(c.name == "write-round-trips" and c.ok for c in report.checks)
+
+
+def test_conformance_write_probe_is_content_preserving(tmp_path):
+    (tmp_path / "Home.md").write_text("keep me", encoding="utf-8")
+    shard = FolderAdapter("w", tmp_path, writable=True)
+    run_conformance(shard)
+    assert shard.read("Home").body == "keep me"  # probe did not alter content