feat(ttp): E.1.11 RuleStore contract — base ABC, factory, filesystem + database stubs

Adds decnet/ttp/store/ subpackage:
- base.py: RuleState frozen dataclass, RuleChange NamedTuple, RuleStore ABC
- factory.py: get_rule_store() reading DECNET_TTP_RULE_STORE_TYPE
- impl/filesystem.py: FilesystemRuleStore with sys.platform=='linux'
  fail-fast guard, allowlist filename regex, raw inotify mask bits
  (lib import deferred to E.3 so contract phase compiles without the
  asyncinotify dep installed)
- impl/database.py: DatabaseRuleStore stub (no platform guard)

TTPRule + TTPRuleState SQLModels were already shipped at E.1.1; this
commit closes the type-only TYPE_CHECKING forward-ref in
rule_engine.py via real runtime imports through the new package.

decnet/ttp/store/__init__.py

@@ -0,0 +1,25 @@
+"""TTP rule store — pluggable backend for rule definitions + state.
+
+Contract step E.1.11 of ``development/TTP_TAGGING.md``. Two backends
+ship:
+
+* :class:`FilesystemRuleStore` — reads ``./rules/ttp/`` at projroot,
+  inotify-watches for hot-reload, holds operational state in-process.
+  Linux-only (the inotify dependency is non-portable by design).
+* :class:`DatabaseRuleStore` — mirrors rule content into ``ttp_rule``
+  with state in ``ttp_rule_state``; survives restart and propagates
+  to every worker in a swarm.
+
+Selection via ``DECNET_TTP_RULE_STORE_TYPE`` (default ``"filesystem"``).
+"""
+from __future__ import annotations
+
+from decnet.ttp.store.base import RuleChange, RuleState, RuleStore
+from decnet.ttp.store.factory import get_rule_store
+
+__all__ = [
+    "RuleChange",
+    "RuleState",
+    "RuleStore",
+    "get_rule_store",
+]