feat: Rust testing framework with mix test integration ($350 bounty #102)

[JirA44]

Summary

• Adds Lux.Rust.Testing — a structured Elixir interface for running cargo test, parsing output, measuring coverage, and managing temporary Rust fixtures
• Adds Mix.Tasks.Rust.Test (mix rust.test) that discovers every Cargo.toml in the repo, runs tests in each project, and exits non-zero on failures — integrating cleanly with CI pipelines
• Adds a comprehensive unit-test suite (test/unit/lux/rust/testing_test.exs) that covers output parsing with sample strings (no real Cargo needed), fixture lifecycle, and result struct shape

What's in each file

lib/lux/rust/testing.ex

Function	Description
run_tests/2	cargo test → {:ok, result_map}
run_tests!/2	Same, raises on failure
run_test/3	Run a single named test
with_coverage/2	cargo llvm-cov, falls back to cargo test
parse_test_output/1	Pure parser — no subprocess
generate_report/2	HTML or text coverage report via cargo llvm-cov
setup_test_fixture/2	Create temp Cargo project with given source
cleanup_fixture/1	Remove temp project

Result map shape:

%{passed: integer, failed: integer, ignored: integer,
  output: string, duration_ms: integer, coverage_pct: float | nil}

lib/mix/tasks/rust.test.ex

• Recursively finds Cargo.toml files (skipping target/ and workspace-only manifests)
• Accepts --dir, --release, --features flags
• Prints pass/fail per project with ANSI colour, summary line at the end
• Non-zero exit when any suite has failures

test/unit/lux/rust/testing_test.exs

• parse_test_output/1 — 7 tests covering all-pass, failures, ignored, fractional duration, empty input, unparseable input
• setup_test_fixture/2 — 5 tests (directory created, Cargo.toml present, source written, name embedded, uniqueness)
• cleanup_fixture/1 — 2 tests (removes dir, idempotent on missing dir)
• run_tests/2 — skipped when cargo is absent, exercises result map shape when available

Test plan

• mix test test/unit/lux/rust/testing_test.exs passes on machines without Cargo installed (parser tests are pure Elixir)
• mix rust.test exits 0 on a repo with no Rust projects (prints yellow notice)
• mix rust.test correctly finds and runs any Cargo projects that exist
• mix rust.test --release forwards the flag to cargo test
• On a machine with Cargo: setup_test_fixture + run_tests round-trip produces passed >= 1, failed == 0

/opire try #102

[zhaog100]

/opire try

Claiming this bounty. Will deliver a complete implementation.