Run Miri on test suite to detect undefined behavior in unsafe code

[ddsha441981]

Summary

PulseMap contains several unsafe blocks:

• SlabPool (src/engine/slab.rs): Manual heap allocation via std::alloc::Layout, raw pointer reads/writes
• MetaWord (src/engine/meta.rs): AtomicU64 with Relaxed ordering
• PulseMapRaw (src/raw.rs): unsafe impl Send (intentionally no Sync)
• AccessBuffer (src/engine/access_buffer.rs): unsafe impl Send + Sync
• simd.rs: unsafe SSE2 intrinsics

Miri is Rust's official interpreter for detecting undefined behavior (UB) at runtime. Running the existing test suite under Miri can catch memory errors that normal tests miss: use-after-free, out-of-bounds access, invalid pointer alignment, and violations of Rust's aliasing model (Stacked Borrows).

What To Do

1. Try running the existing tests under Miri

rustup +nightly component add miri
cargo +nightly miri test -p pulse_map --no-default-features  # core engine only
cargo +nightly miri test -p pulse_map                        # with std features

2. Document and fix any Miri violations

• If Miri reports UB, create a separate issue for each distinct violation with the full Miri backtrace
• Common findings in projects like this:
  • Stacked Borrows violations from casting &self to *mut (we already fixed the big one in v0.6.2)
  • Alignment issues in repr(C, packed) structs (our Slot is packed)
  • Provenance issues in the SlabPool raw pointer usage

3. Add Miri to CI

Add a new job in .github/workflows/build.yml:

- name: Run Miri
  run: |
    rustup +nightly component add miri
    cargo +nightly miri test -p pulse_map --no-default-features

Known Limitations

• Miri does not support inline assembly or SIMD intrinsics. Tests using the simd feature will need to be skipped (--no-default-features avoids this)
• Miri is slow (~100x slower than normal execution). This is expected.

Acceptance Criteria

• All existing tests pass under cargo +nightly miri test --no-default-features
• Any Miri violations are documented as separate issues with full backtraces
• Miri CI job added to .github/workflows/build.yml
• No changes to core engine logic unless fixing a genuine Miri-detected UB