feat(fusion_accounting_ai): add DataAdapter base + registry

Made-with: Cursor

fusion_accounting_ai/services/data_adapters/_registry.py

@@ -0,0 +1,25 @@
+"""Registry: lazy-loads data adapter instances per env."""
+
+from .base import DataAdapter
+
+
+def get_adapter(env, name: str) -> DataAdapter:
+    """Return a data adapter by short name. Cached per request via env.context."""
+    cache = env.context.get('_fusion_data_adapter_cache')
+    if cache is None:
+        cache = {}
+    if name not in cache:
+        cls = _ADAPTERS.get(name)
+        if cls is None:
+            raise KeyError(f"Unknown data adapter: {name!r}. Known: {list(_ADAPTERS)}")
+        cache[name] = cls(env)
+    return cache[name]
+
+
+# Populated as adapter classes are added (Tasks 9, 10, 11).
+_ADAPTERS: dict[str, type[DataAdapter]] = {}
+
+
+def register_adapter(name: str, cls: type[DataAdapter]) -> None:
+    """Register an adapter class. Call from each adapter module at import time."""
+    _ADAPTERS[name] = cls