feat: Rust code quality and website improvements for Apache Mahout (QDP & site)

[0lai0]

Related Issues

Closes #1080

Changes

• Bug fix
• New feature
• Refactoring
• Documentation
• Test
• CI/CD pipeline
• Other

Why

The QDP Rust core (qdp-core) currently has several broad unsafe blocks without safety documentation, lacks a CI pipeline to enforce rustdoc quality, and has multiple TODO placeholders across the documentation site.

How

1. Rust Code Quality (Unsafe Refactoring & Documentation):

• dlpack.rs : Added module-level documentation (//!) and // SAFETY: explanations for all FFI pointer lifecycles and CUDA stream synchronizations.

• gpu/memory.rs : Narrowed unsafe scopes (e.g., query_cuda_mem_info , PinnedHostBuffer::new), added // SAFETY: comments for Send/Sync traits, and replaced eprintln! with standard log::warn! in the Drop implementation.

• gpu/pipeline.rs : Optimized high-frequency environment variable reads using std::sync::OnceLock and narrowed unsafe blocks for CUDA event creation/destruction.

• error.rs: Formatted MahoutError variants with RFC 505 compliant rustdoc.

• lib.rs & cuda_ffi.rs: Added comprehensive crate-level and module-level architecture documentation.

2. Test Coverage Planning:

• Approximately 25 deterministic unit test cases have been outlined with TODO annotations to ensure logic coverage in files that currently lack tests.

3. CI/CD Pipeline:
   .github/workflows/rustdoc.yml
   to run cargo doc --no-deps with RUSTDOCFLAGS="-D warnings" on PRs, acting as a quality gate against documentation drift.

4. Website Documentation:

• Replaced TODO placeholders in docs/qumat/examples.md with 3 functional QuMat Python examples (Bell state, multiple backends, rotation gates).

• Replaced TODO placeholders in docs/qumat/concepts.md with core QDP integration concepts.

• Created docs/contributing/rust-docs.md to establish project standards for RFC 505 formatting and // SAFETY: comment conventions.