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>

src/shard_wiki/adapters/conformance.py

@@ -98,6 +98,22 @@ def run_conformance(adapter: ShardAdapter) -> ConformanceReport:
     if profile.supports(Verb.READ):
         checks.append(_safe(_read_round_trips, "read-round-trips"))
 
+    # WRITE positive probe: a claimed-writable shard must actually round-trip a write. The probe
+    # is content-preserving (rewrite an existing page with its own body) so it is non-destructive.
+    def _write_round_trips() -> Check:
+        keys = list(adapter.keys())
+        if not keys:
+            return Check("write-round-trips", True, "empty shard")
+        k = keys[0]
+        original = adapter.read(k).body
+        adapter.write(k, original)
+        if adapter.read(k).body != original:
+            return Check("write-round-trips", False, "rewrite did not preserve body")
+        return Check("write-round-trips", True)
+
+    if profile.supports(Verb.WRITE):
+        checks.append(_safe(_write_round_trips, "write-round-trips"))
+
     # Honest absence: an *unclaimed* optional verb must raise NotSupported when invoked.
     for verb in _HONEST_ABSENCE_VERBS:
         if profile.supports(verb):

src/shard_wiki/adapters/folder.py

@@ -23,6 +23,7 @@ from shard_wiki.model import (
     Identity,
     MergeModel,
     NativeQuery,
+    NotSupported,
     OperationalEnvelope,
     Page,
     Placement,
@@ -37,19 +38,22 @@ __all__ = ["FolderAdapter"]
 
 
 class FolderAdapter(ShardAdapter):
-    def __init__(self, shard_id: str, root: str | Path) -> None:
+    def __init__(self, shard_id: str, root: str | Path, writable: bool = False) -> None:
         self._shard_id = shard_id
         self._root = Path(root)
+        self._writable = writable
 
     @property
     def shard_id(self) -> str:
         return self._shard_id
 
     def profile(self) -> CapabilityProfile:
+        verbs = {Verb.READ, Verb.WRITE} if self._writable else {Verb.READ}
+        granularity = WriteGranularity.PER_PAGE if self._writable else WriteGranularity.NONE
         return CapabilityProfile(
             substrate=Substrate.FILES,
             attachment_mode=AttachmentMode.FILE_STORE,
-            write_granularity=WriteGranularity.NONE,
+            write_granularity=granularity,
             content_opacity=ContentOpacity.TRANSPARENT,
             operational_envelope=OperationalEnvelope.LOCAL_UNBOUNDED,
             access_grant=AccessGrant.OPEN,
@@ -59,12 +63,28 @@ class FolderAdapter(ShardAdapter):
             addressing=Addressing.PATH,
             native_query=NativeQuery.NONE,
             translation=Translation.NATIVE,
-            supported_verbs=frozenset({Verb.READ}),
+            supported_verbs=frozenset(verbs),
         ).validate()
 
     def _path_for(self, key: str) -> Path:
         return self._root / f"{key}.md"
 
+    def current_rev(self, key: str) -> str | None:
+        """The shard's current revision token for ``key`` (mtime iso), or ``None`` if absent.
+        Used for apply-under-drift comparison (blueprint §8.6)."""
+        path = self._path_for(key)
+        if not path.is_file():
+            return None
+        return datetime.fromtimestamp(path.stat().st_mtime, tz=timezone.utc).isoformat()
+
+    def write(self, key: str, body: str) -> Page:
+        if not self._writable:
+            raise NotSupported(f"{type(self).__name__} is read-only")
+        path = self._path_for(key)
+        path.parent.mkdir(parents=True, exist_ok=True)
+        path.write_text(body, encoding="utf-8")
+        return self.read(key)
+
     def keys(self) -> Iterable[str]:
         for p in sorted(self._root.rglob("*.md")):
             yield p.relative_to(self._root).with_suffix("").as_posix()