Add inline CTable support to TreeStore

   TreeStore can now store CTable objects as first-class inline leaves
   alongside NDArrays:

       ts["/arr"] = blosc2.arange(10)
       ts["/table"] = ctable          # CTable stored inline
       table = ts["/table"]           # returns CTable transparently

   Physical layout: CTable internals (_meta, _valid_rows, _cols/*, _indexes/*)
   are stored as ordinary Blosc2 leaves inside the outer store's working
   directory, avoiding nested ZIPs and keeping everything directly addressable
   by offset in .b2z bundles.

   Implementation:
   - Add TreeStoreTableStorage backend in ctable_storage.py that maps CTable
     logical keys onto an outer TreeStore's map_tree/working_dir
   - Refactor CTable.save() and CTable.open() around shared _save_to_storage()
     and _open_from_storage() helpers; add private _save_to_treestore() and
     _open_from_treestore() used by TreeStore
   - Add persistent object registry in TreeStore (embed-store vlmeta) to
     track object roots; probe _/<key>/_meta as fallback for old stores
   - Update TreeStore.__setitem__ to dispatch CTable and block writes to
     object internals
   - Update TreeStore.__getitem__ to return CTable for registered object roots
   - Update TreeStore.__delitem__ to remove all physical leaves of an object
     root and unregister it; block direct deletion of internals
   - Update keys(), __contains__, walk(), get_children(), get_descendants()
     to treat object roots as opaque leaves and hide their internals
   - get_subtree() raises ValueError on object root paths
   - TreeStore.close()/discard() flush inline CTable handles before repacking
   - Add 30 new tests covering b2d/b2z, append mode, traversal, guards,
     deletion, roundtrips, and string columns
   - Add plans/tree_store_ctable_ndarray.md with design rationale

Author: FrancescAlted