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>

tests/test_engine_activation.py

@@ -0,0 +1,61 @@
+"""Tests for per-shard extension activation (SHARD-WP-0014 T3, ADR-0001)."""
+
+from shard_wiki.engine import (
+    ActivationContext,
+    ActivationResolver,
+    StaticProvider,
+    feature_control_provider,
+)
+
+CANDIDATES = ["ext.overlay", "ext.views", "ext.struct", "ext.compute"]
+
+
+def test_static_provider_default_off():
+    r = ActivationResolver(StaticProvider())  # nothing enabled
+    assert r.active_extensions(CANDIDATES, ActivationContext("s")) == set()
+
+
+def test_static_provider_global_flags():
+    r = ActivationResolver(StaticProvider(flags={"ext.overlay": True, "ext.views": True}))
+    assert r.active_extensions(CANDIDATES, ActivationContext("s")) == {"ext.overlay", "ext.views"}
+
+
+def test_per_shard_scoping_overrides_global():
+    provider = StaticProvider(
+        flags={"ext.views": True},
+        per_shard={"engB": {"ext.struct": True, "ext.views": False}},
+    )
+    r = ActivationResolver(provider)
+    assert r.active_extensions(CANDIDATES, ActivationContext("engA")) == {"ext.views"}
+    assert r.active_extensions(CANDIDATES, ActivationContext("engB")) == {"ext.struct"}
+
+
+def test_context_carries_tenant():
+    captured = {}
+
+    class Spy(StaticProvider):
+        def is_active(self, feature_key, context):
+            captured.update(context)
+            return super().is_active(feature_key, context)
+
+    ActivationResolver(Spy(flags={"ext.views": True})).active_extensions(
+        ["ext.views"], ActivationContext("s1", tenant_id="acme")
+    )
+    assert captured["shard_id"] == "s1" and captured["tenant_id"] == "acme"
+
+
+def test_activation_profile_returns_config_for_active():
+    provider = StaticProvider(
+        flags={"ext.struct": True},
+        configs={"ext.struct": {"max_fields": 50}},
+    )
+    profile = ActivationResolver(provider).activation_profile(CANDIDATES, ActivationContext("s"))
+    assert profile == {"ext.struct": {"max_fields": 50}}
+
+
+def test_feature_control_provider_degrades_gracefully():
+    # feature_control_sdk is not a dependency of shard-wiki: when absent the factory returns
+    # None (standalone path stays dependency-free, ADR-0001); when present it yields a usable
+    # ActivationProvider. Either way it must not raise.
+    provider = feature_control_provider()
+    assert provider is None or hasattr(provider, "is_active")