Lift ProgrammingError import + prune aio.cursor __all__

Cycle 7 cleanups:
- AsyncCursor.arraysize.setter imported ProgrammingError inside the
  function body on every call. Moved to module-level imports to
  match the sync Cursor and avoid repeated runtime import work.
- _strip_leading_comments was leaked into aio.cursor's __all__ when
  ISSUE-13 consolidated the helper. It's a private utility; dropped
  from __all__ so IDE/grep don't surface it as public API.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: antoineleclair

src/dqlitedbapi/aio/cursor.py

@@ -9,13 +9,13 @@
     _convert_row,
     _strip_leading_comments,
 )
-from dqlitedbapi.exceptions import InterfaceError
+from dqlitedbapi.exceptions import InterfaceError, ProgrammingError
 
 if TYPE_CHECKING:
     from dqlitedbapi.aio.connection import AsyncConnection
 
 
-__all__ = ["AsyncCursor", "_strip_leading_comments"]
+__all__ = ["AsyncCursor"]
 
 
 class AsyncCursor:
@@ -62,8 +62,6 @@ def arraysize(self) -> int:
     @arraysize.setter
     def arraysize(self, value: int) -> None:
         if value < 1:
-            from dqlitedbapi.exceptions import ProgrammingError
-
             raise ProgrammingError(f"arraysize must be >= 1, got {value}")
         self._arraysize = value