[+] add ORM registry with migration support, migrate cache/db.py
1. add apps/orm/registry.py with orm_module_t base class and orm_registry_t singleton;
2. singleton per db path, thread-safe, tracks registered ORM classes;
3. orm_schema_versions table for per-module version tracking;
4. classmethods table_prefix(), schema_version(), migrate() for schema management;
5. registry.module(cls) returns typed ORM instance, cached per registry;
6. migrate cache_db_t to extend orm_module_t, move schema into classmethod migrate();
7. cache_db_t constructor accepts Path (legacy, uses registry) or Connection (from registry);
8. orm_registry_t.register(cache_db_t) at module load time;
9. add test_orm.py with 12 tests: singleton, migration, multi-module, incremental, failure;
LLM
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