Add Python CI, update API docs, and fix Rust append behavior

[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 updates CI and documentation while addressing a Rust Qube::append edge case when appending into an empty receiver.

Changes:

• Add an empty-receiver fast-path to Qube::append and a regression test covering append-to-empty behavior.
• Expand Rust and Python API documentation to include additional manipulation methods (drop, squeeze, etc.).
• Update GitHub Actions CI to run Python binding tests inside an isolated uv-managed virtual environment.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
qubed/src/merge.rs	Adds an is_empty() fast-path for append by copying other into self and clearing other.
qubed/tests/test_union.rs	Adds a test for appending into an empty Qube; removes commented-out code.
docs/src/rust/qubed.md	Documents additional Rust API methods and adds examples for drop/squeeze.
docs/src/python/py_qubed.md	Adds Python docs for compress, drop, squeeze, and query methods.
.github/workflows/ci.yml	Runs Python tests in a dedicated uv venv rather than the global runner environment.

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