feat(engine): typed-extension runtime (WP-0014 T2)

engine/extension.py: Extension contract (id/provides/declares_types/depends_on/
conflicts_with + bound hooks), ExtensionRuntime (register-with-verification,
activate = dependency closure + conflict + type-collision checks rejecting
impossible profiles), and ActiveExtensions with deterministic (topological, id-tie)
hook dispatch — transform hooks chain, collect hooks gather. 9 tests green,
coverage floor held, pyflakes clean. Marks T2 done.

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

src/shard_wiki/engine/__init__.py

@@ -5,7 +5,22 @@ is *one shard*: it is consumed by the orchestrator only via its `EngineShardAdap
 imports the derived tier (`union`/`projection`).
 """
 
+from shard_wiki.engine.extension import (
+    ActiveExtensions,
+    Extension,
+    ExtensionError,
+    ExtensionRuntime,
+    Hook,
+)
 from shard_wiki.engine.kernel import EngineKernel
 from shard_wiki.engine.links import extract_wikilinks
 
-__all__ = ["EngineKernel", "extract_wikilinks"]
+__all__ = [
+    "EngineKernel",
+    "extract_wikilinks",
+    "Hook",
+    "Extension",
+    "ExtensionError",
+    "ExtensionRuntime",
+    "ActiveExtensions",
+]