feat(engine): per-shard extension activation (WP-0014 T3, ADR-0001)

engine/activation.py: ActivationContext (shard/tenant, no authz), pluggable
ActivationProvider protocol, StaticProvider standalone default (zero-dep, global
flags + per-shard scoping + per-ext config), ActivationResolver (candidate ids ->
active set / activation profile), and feature_control_provider() lazy factory
(returns None when feature_control_sdk absent -> degrade to static; OpenFeature-
shaped when present). Availability only. 6 tests green, coverage held, pyflakes clean.

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

src/shard_wiki/engine/__init__.py

@@ -5,6 +5,13 @@ is *one shard*: it is consumed by the orchestrator only via its `EngineShardAdap
 imports the derived tier (`union`/`projection`).
 """
 
+from shard_wiki.engine.activation import (
+    ActivationContext,
+    ActivationProvider,
+    ActivationResolver,
+    StaticProvider,
+    feature_control_provider,
+)
 from shard_wiki.engine.extension import (
     ActiveExtensions,
     Extension,
@@ -23,4 +30,9 @@ __all__ = [
     "ExtensionError",
     "ExtensionRuntime",
     "ActiveExtensions",
+    "ActivationContext",
+    "ActivationProvider",
+    "StaticProvider",
+    "ActivationResolver",
+    "feature_control_provider",
 ]

src/shard_wiki/engine/activation.py

@@ -0,0 +1,129 @@
+"""Per-shard extension activation (WikiEngineCoreArchitecture E-4/E-8, ADR-0001).
+
+Decides *which registered extensions are active* for a given shard — **availability only, never
+authorization**. The mechanism is OpenFeature-shaped and **provider-pluggable**:
+
+- :class:`StaticProvider` is the **standalone / L0 default** — zero external dependency, in-process
+  flags with optional per-shard scoping. A kernel-only or offline shard uses this.
+- :func:`feature_control_provider` lazily wraps the helix_forge ``feature_control_sdk`` (over
+  OpenFeature) when it is installed; absent, it returns ``None`` and the caller falls back to the
+  static default. This mirrors shard-wiki's identity-provider ladder (local default → external
+  when present), and keeps the engine core pure-stdlib.
+
+An *activation profile* is ``{extension id → config}``; the active id set then feeds the
+extension runtime's `activate()` (T2) and the derived capability profile (T4, E-5).
+"""
+
+from __future__ import annotations
+
+from collections.abc import Iterable, Mapping
+from dataclasses import dataclass, field
+from typing import Any, Protocol, runtime_checkable
+
+__all__ = [
+    "ActivationContext",
+    "ActivationProvider",
+    "StaticProvider",
+    "ActivationResolver",
+    "feature_control_provider",
+]
+
+
+@dataclass(frozen=True, slots=True)
+class ActivationContext:
+    """Scope for an activation decision. Carries no principal/authz — availability only."""
+
+    shard_id: str
+    tenant_id: str | None = None
+    extra: Mapping[str, Any] = field(default_factory=dict)
+
+    def as_dict(self) -> dict[str, Any]:
+        d: dict[str, Any] = {"shard_id": self.shard_id}
+        if self.tenant_id is not None:
+            d["tenant_id"] = self.tenant_id
+        d.update(self.extra)
+        return d
+
+
+@runtime_checkable
+class ActivationProvider(Protocol):
+    """Evaluates feature availability for an extension key in a context (OpenFeature-shaped)."""
+
+    def is_active(self, feature_key: str, context: Mapping[str, Any]) -> bool: ...
+
+    def config(self, feature_key: str, context: Mapping[str, Any]) -> Mapping[str, Any]: ...
+
+
+@dataclass(frozen=True, slots=True)
+class StaticProvider:
+    """The standalone default: in-process flags, optionally overridden per shard. Zero deps.
+
+    ``flags`` is the base availability map; ``per_shard[shard_id]`` overrides it for one shard;
+    ``configs[feature_key]`` supplies per-extension config. Unknown keys → ``default``.
+    """
+
+    flags: Mapping[str, bool] = field(default_factory=dict)
+    per_shard: Mapping[str, Mapping[str, bool]] = field(default_factory=dict)
+    configs: Mapping[str, Mapping[str, Any]] = field(default_factory=dict)
+    default: bool = False
+
+    def is_active(self, feature_key: str, context: Mapping[str, Any]) -> bool:
+        shard = context.get("shard_id")
+        scoped = self.per_shard.get(shard, {}) if shard is not None else {}
+        if feature_key in scoped:
+            return scoped[feature_key]
+        return self.flags.get(feature_key, self.default)
+
+    def config(self, feature_key: str, context: Mapping[str, Any]) -> Mapping[str, Any]:
+        return self.configs.get(feature_key, {})
+
+
+class ActivationResolver:
+    """Maps candidate extension ids → the active set / activation profile for a context."""
+
+    def __init__(self, provider: ActivationProvider) -> None:
+        self.provider = provider
+
+    def active_extensions(
+        self, candidate_ids: Iterable[str], context: ActivationContext
+    ) -> set[str]:
+        ctx = context.as_dict()
+        return {eid for eid in candidate_ids if self.provider.is_active(eid, ctx)}
+
+    def activation_profile(
+        self, candidate_ids: Iterable[str], context: ActivationContext
+    ) -> dict[str, Mapping[str, Any]]:
+        """``{extension id → config}`` for the active subset."""
+        ctx = context.as_dict()
+        return {
+            eid: self.provider.config(eid, ctx)
+            for eid in candidate_ids
+            if self.provider.is_active(eid, ctx)
+        }
+
+
+def feature_control_provider(domain: str | None = None) -> ActivationProvider | None:
+    """Return a feature-control-backed provider if ``feature_control_sdk`` is importable, else
+    ``None`` (caller falls back to :class:`StaticProvider`). Lazy import keeps the engine core
+    dependency-free (ADR-0001)."""
+    try:  # optional engine extra — not a core dependency
+        from feature_control_sdk import FeatureControlClient  # type: ignore
+    except ImportError:
+        return None
+
+    client = FeatureControlClient(domain=domain)
+
+    @dataclass(frozen=True, slots=True)
+    class _FeatureControlProvider:
+        _client: Any
+
+        def is_active(self, feature_key: str, context: Mapping[str, Any]) -> bool:
+            return bool(
+                self._client.get_boolean_value(feature_key, False, context=dict(context))
+            )
+
+        def config(self, feature_key: str, context: Mapping[str, Any]) -> Mapping[str, Any]:
+            getter = getattr(self._client, "get_object_value", None)
+            return dict(getter(feature_key, {}, context=dict(context))) if getter else {}
+
+    return _FeatureControlProvider(client)