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.