Incrementally integrate external Rust test suites into mir-semantics CI

Context

PR #962 (jh/test_coverage_20260225_master_pr) explored adding external test coverage infrastructure — coverage matrix, fetch scripts, batch runner, report tooling, and test harnesses for 5 external suites — all at once. That approach is too large for a single PR. We will add tests incrementally instead.

External test suites evaluated

TODO

• Import stable-mir-json UI tests as mir-semantics tests
• Add passing miri-pass tests to prove-rs suite
• Add passing miri-fail tests to prove-rs suite
• Add passing ui-run-pass tests to prove-rs suite
• Add passing kani tests to prove-rs suite
• Investigate rustlantis generated programs for potential test coverage

Workflow

For each test being added:

1. Verify stable-mir-json can compile the Rust file and produce valid SMIR JSON
2. Add the test to mir-semantics prove-rs / integration tests
3. Keep PRs small — a handful of tests from one suite per PR

Reference

• PR #962 contains the original analysis and infrastructure prototype (closed without merge)

Start with PR #962 and the stable-mir-json workflow, then choose one external suite from the TODO list. Check the relevant prove-rs or integration tests and verify that stable-mir-json compiles each selected Rust file into valid SMIR JSON. Done means a small PR adds a handful of tests from one suite and they pass in mir-semantics CI.