feat(ci): Add Miri to CI workflow to verify absence of Undefined Behavior

[kalwalt]

Summary

Add Miri as a GitHub Actions CI job to detect undefined behavior (UB) in the unsafe code paths at test-time.

Motivation

PureCV strives for a zero-unsafe policy, but a few unsafe blocks currently exist for performance-critical SIMD paths:

• src/core/arithm.rs — pointer casts in binary_op! / binary_op_scalar! macros (transmuting dst chunks)
• src/imgproc/derivatives.rs — from_raw_parts / from_raw_parts_mut reinterpretation for f32 SIMD paths
• src/core/tests.rs — raw pointer dereference in data_ptr / data_ptr_mut tests
  Requirements
  During development, a plan must be created identifying which code/tests to include and exclude from Miri runs:

Code to Include (run under Miri)

• All unit tests in src/core/tests.rs (especially data_ptr and data_ptr_mut tests)
• Arithmetic operations using unsafe transmutes (src/core/arithm.rs)
• Derivative SIMD fast-path (src/imgproc/derivatives.rs)
• src/core/structural.rs split/merge operations
• All safe code paths (sequential/non-SIMD) — should pass trivially

Code to Exclude (skip under Miri)

• Benchmarks (benches/) — too slow under Miri interpretation
• WASM build (crates/wasm/) — Miri does not support wasm32 targets
• pulp-based SIMD (#[cfg(feature = "simd")]) — Miri doesn't support most SIMD intrinsics; annotate with #[cfg_attr(miri, ignore)]
• Rayon parallel tests (#[cfg(feature = "parallel")]) — Miri's thread support is limited; run without parallel feature or selectively ignore
• External I/O or FFI — none expected, but exclude if added

Implementation Plan

1. Add a new job miri to .github/workflows/ci.yml

2. Use dtolnay/rust-toolchain@nightly with components: miri

3. Run: cargo +nightly miri test --workspace (without parallel or simd features initially)

4. Annotate incompatible tests with #[cfg_attr(miri, ignore)]

5. Consider a second Miri run with --features simd once pulp gains Miri compatibility

6. Document excluded tests and rationale in the workflow file

7. Add a Badge in the readme file with all infos.

Proposed Workflow Addition

    name: Miri UB Check
    runs-on: ubuntu-latest
    steps:
    - uses: actions/checkout@v6
    - name: Install Rust nightly + Miri
      uses: dtolnay/rust-toolchain@nightly
      with:
        components: miri
    - name: Rust Cache
      uses: Swatinem/rust-cache@v2
    - name: Run Miri
      run: cargo +nightly miri test --workspace
      env:
        MIRIFLAGS: "-Zmiri-strict-provenance"

Acceptance Criteria

• Miri CI job passes on the dev branch
• All existing unsafe code is verified UB-free (or documented exceptions)
• Tests incompatible with Miri are annotated with #[cfg_attr(miri, ignore)]
• Plan document exists identifying included/excluded code with rationale