feat(adapters): ShardAdapter contract + read-only FolderAdapter (WP-0007 T3)

Versioned ShardAdapter ABC (shard_id/profile/keys/read mandatory; optional verbs
raise NotSupported by default = honest absence). FolderAdapter reads a dir of
Markdown into Pages (relpath=key, mtime=source_rev, provenance envelope),
declaring a validated read-only file-store profile. 4 tests green.

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

src/shard_wiki/adapters/contract.py

@@ -0,0 +1,52 @@
+"""The shard adapter contract — the bottom narrow waist (CoreArchitectureBlueprint §6, TSD §A).
+
+A backend participates by implementing :class:`ShardAdapter`. ``shard_id``, ``profile`` and
+``read`` are mandatory; everything else is an optional capability that defaults to raising
+:class:`~shard_wiki.model.NotSupported` — so a limited backend is honest about what it can't do
+(graceful degradation, I-8) and core never assumes a capability it wasn't given (capability-as-
+data, I-3). Declared profiles are verified by the conformance suite (T4), never taken on trust.
+"""
+
+from __future__ import annotations
+
+import abc
+from collections.abc import Iterable
+
+from shard_wiki.model import CapabilityProfile, NotSupported, Page
+
+__all__ = ["ShardAdapter", "CONTRACT_VERSION"]
+
+CONTRACT_VERSION = "0.1"
+
+
+class ShardAdapter(abc.ABC):
+    """Versioned interface a backend implements to attach as a shard."""
+
+    contract_version: str = CONTRACT_VERSION
+
+    @property
+    @abc.abstractmethod
+    def shard_id(self) -> str:
+        """Stable id scoping every Identity this shard mints."""
+
+    @abc.abstractmethod
+    def profile(self) -> CapabilityProfile:
+        """The (to-be-verified) capability profile of this binding."""
+
+    @abc.abstractmethod
+    def keys(self) -> Iterable[str]:
+        """The stable page keys this shard offers (the handle half of Identity)."""
+
+    @abc.abstractmethod
+    def read(self, key: str) -> Page:
+        """Read one page by its stable key. Raises ``KeyError`` if absent."""
+
+    # --- optional capability verbs: honest NotSupported by default ---
+    def write(self, key: str, body: str) -> Page:  # noqa: ARG002
+        raise NotSupported(f"{type(self).__name__} does not support write")
+
+    def diff(self, key: str, other: str) -> str:  # noqa: ARG002
+        raise NotSupported(f"{type(self).__name__} does not support diff")
+
+    def notify(self):
+        raise NotSupported(f"{type(self).__name__} does not support notify")