Validate wheel installs in CI

[Xuanwo]

This changes CI to validate the actual wheel install path instead of the source tree. The Python workflow now builds the root and backend wheels once, reuses those artifacts for a clean-environment loader test and a GPU wheel smoke test, and updates the local just tasks and README around the same flow.

It also narrows the Rust-only workflow to fmt, clippy, and cargo check, removes the separate source-path GPU workflow, and keeps python-release.yml in place for PyPI trusted publishing while limiting it to the minimal build-and-publish path required by the OIDC setup. The local checks I ran were just loader-test, just --dry-run python-build, workflow YAML parsing, and git diff --check.