feat: introduce Module framework for persistent storage modules

[zhanglei1949]

Summary

Adds the Module abstract interface, ModuleFactory singleton registry, and ModuleDescriptor metadata struct — infrastructure for the upcoming checkpoint / snapshot store work to persist and restore extensible storage modules in a type-erased way.

This PR is pure addition with no caller. Reviewers are asked to look at the API shape rather than usage; the callers (the four Module-inheriting base classes and the Checkpoint / SnapshotStore that drive them) land in the follow-up workspace / checkpoint PR.

This is PR-B1 of the #370 split — it can be reviewed in parallel with #401 (PR-A: storage view layer refactor), since the two are independent.

What's in the framework

• Module (include/neug/storages/module/module.h) — abstract base with four lifecycle hooks: Open(Checkpoint&, ModuleDescriptor&, MemoryLevel), Dump(Checkpoint&) -> ModuleDescriptor, Close(), Fork(Checkpoint&, MemoryLevel) -> unique_ptr<Module>. Both Dump and Fork are expected to write into a UUID sub-directory under the checkpoint runtime dir.
• ModuleFactory (include/neug/storages/module/module_factory.h, src/storages/module/module_factory.cc) — singleton registry mapping module_type strings to creator functions. NEUG_REGISTER_MODULE(Class) / NEUG_REGISTER_TEMPLATE_MODULE(Tmpl, T) macros perform static-init registration; the template variant uses __COUNTER__ so types containing :: (e.g. std::string_view) still get a stable unique function identifier.
• ModuleDescriptor (include/neug/storages/module_descriptor.h, src/storages/graph/module_descriptor.cc) — metadata struct round-trippable through rapidjson, carrying path / size / module_type plus arbitrary extra KV pairs and recursive sub-module descriptors. Pimpl is used for the sub-module map to avoid the incomplete-type issue of a self-referential unordered_map inside the struct definition.
• StorageTypeName<T> (include/neug/storages/module/type_name.h) — trait mapping storage value types (int32_t, std::string_view, Date, ...) to short factory key suffixes.
• UUIDGenerator (include/neug/utils/uuid.h, src/utils/uuid.cc) — small RFC-4122-style UUID generator used to name per-Dump / per-Fork sub-directories.

Wiring

• src/storages/module/CMakeLists.txt — new neug_storages_module OBJECT library (GLOB).
• src/storages/CMakeLists.txt — adds add_subdirectory(module) and wires the new OBJECT files into NEUG_STORAGES_OBJFILES.
• module_descriptor.cc is placed under src/storages/graph/ (and picked up by the existing GLOB there) because it instantiates the pimpl that wraps a self-referential map of ModuleDescriptors — keeping it in the same translation unit family avoids a circular dependency between neug_storages_module and neug_property_graph.
• uuid.cc is placed under src/utils/ (existing GLOB picks it up).

No changes to existing files other than the two-line src/storages/CMakeLists.txt edit.

Review focus

• API shape: are the four Module lifecycle hooks the right surface, given they will be driven by Checkpoint / SnapshotStore in the follow-up?
• ModuleDescriptor shape: extra_ (free-form KV) + sub_modules_ (recursive) + JSON round-trip is enough metadata for the downstream callers; reviewers may want to push back if a flatter shape would do.
• Registration macros: __attribute__((constructor))-based static-init registration is intentional (so callers don't need an explicit init step). The __COUNTER__ indirection in NEUG_REGISTER_TEMPLATE_MODULE is for ::-containing type names — please double-check it works on your toolchain.

Fixes #408