Feat/fix select

[mathleur]

Description

Contributor Declaration

By opening this pull request, I affirm the following:

• All authors agree to the Contributor License Agreement.
• The code follows the project's coding standards.
• I have performed self-review and added comments where needed.
• I have added or updated tests to verify that my changes are effective and functional.
• I have run all existing tests and confirmed they pass.

[Copilot]

Pull request overview

This PR adjusts Qube::select behavior to prune empty placeholder branches when a selected dimension/value only appears deeper in the tree, and updates/extends tests to reflect the corrected semantics (including post-union compression behavior).

Changes:

• Prune newly-created result nodes during selection when recursion yields no matching descendants (avoids empty branches).
• Add Rust and Python test coverage for selecting deep keys and irregular dimension ordering.
• Update union test expectations to match append()’s always-compress behavior.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 5 comments.

File	Description
qubed/src/select.rs	Adds pruning of empty result branches during select_recurse and introduces several regression tests around deep-key selection and irregular trees.
qubed/tests/test_union.rs	Updates test to assert post-append() compressed structure rather than structural hash equality.
py_qubed/tests/test_select_api.py	Adds Python regression tests ensuring deep-key selection prunes non-matching branches; minor formatting cleanups.

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