Incrementally integrate external Rust test suites into mir-semantics CI

[Stevengre]

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

Suite	Source
miri-pass	rust-lang/miri tests/pass/
miri-fail	rust-lang/miri tests/fail/
ui-run-pass	rust-lang/rust tests/ui/ (run-pass)
kani	model-checking/kani tests/kani/
rustlantis	cbeuw/rustlantis (generated programs)

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 test: external suite coverage infrastructure and execution hardening #962 contains the original analysis and infrastructure prototype (closed without merge)