Add GitHub-hosted CPU build and test CI

[voltjia]

Problem

The repository only runs formatting checks. Builds and runtime smoke tests can regress without any required CI signal.

Scope

• Add a separate GitHub-hosted CPU build/test workflow.
• Pin the runner, actions, Python tooling, PyTorch wheel, and InfiniRT commit.
• Build and install InfiniRT with WITH_CPU=ON from public source.
• Build a torch-infini wheel with --no-build-isolation --no-deps.
• Install and test the wheel outside the source tree and verify automatic CPU fallback without runtime configuration.
• Keep the existing lint workflow separate and do not publish the wheel.

Acceptance criteria

• The workflow needs no secrets, private image, or host path.
• Import, automatic backend selection, allocation/copy smoke tests, and the full test suite pass on CPU.
• The workflow is suitable as a required pull-request check.

Depends on #2.