TreeStore to accept both CTable and NDArray as leaves

[FrancescAlted]

This PR makes CTable objects first-class high-level leaf objects in TreeStore.

A TreeStore can now contain regular Blosc2 leaves, such as NDArray, together with inline CTable objects:

with blosc2.TreeStore("bundle.b2z", mode="w") as ts:
    ts["/x"] = blosc2.arange(10)
    ts["/table"] = table

with blosc2.open("bundle.b2z", mode="r") as ts:
    x = ts["/x"]          # NDArray
    table = ts["/table"]  # CTable

[Copilot]

Pull request overview

This PR extends blosc2.TreeStore so a single bundle can store both array leaves (e.g., NDArray) and higher-level table objects (CTable) as collapsed “object roots”, with TreeStore traversal/deletion semantics treating a CTable as a single key while storing its physical components inline under that key.

Changes:

• Add inline CTable support to TreeStore via an object-root registry + fallback probing of physical /_meta manifests, plus protections to hide/block direct access to object internals.
• Refactor CTable persistence/opening around a storage-backend helper and add a TreeStoreTableStorage backend for inline storage under a TreeStore subtree.
• Add tests, examples, and documentation demonstrating mixed NDArray/CTable bundles and expected traversal/deletion behavior.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/blosc2/tree_store.py	Adds object-root registry/probing, CTable dispatch on get/set/del, hides internals in traversal, and manages inline object handles on close/discard.
src/blosc2/ctable_storage.py	Introduces TreeStoreTableStorage to persist CTable components inline inside an outer TreeStore; adjusts index-catalog path handling helpers.
src/blosc2/ctable.py	Refactors open/save paths via _open_from_storage/_save_to_storage and adds private TreeStore inline save/open hooks.
src/blosc2/indexing.py	Avoids reopening .b2z bundle urlpaths for offset-backed leaves during bucket worker setup.
tests/test_tree_store.py	Adds comprehensive tests for mixed NDArray/CTable storage, traversal hiding, conflict rules, deletion semantics, append mode, and b2d/b2z roundtrips.
examples/tree-store.py	Updates example to demonstrate storing, appending to, and deleting an inline CTable inside a TreeStore.
doc/getting_started/tutorials/13.containers.ipynb	Updates tutorial to document inline CTable support in TreeStore and provides a walkthrough example.
plans/tree_store_ctable_ndarray.md	Adds a detailed design/plan document describing the inline-subtree approach, registry, semantics, and test plan.

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