test: preserve math placeholders through Phase A reflow

[1sakshm]

Fixes #97

Summary

• add a Rust regression test proving inline math-placeholder comments survive Phase A paragraph reflow byte-for-byte
• document the ordering/lookup contract in docs/pipeline.md
• make the CUDA-backed smoke module skip cleanly when optional dependencies or CUDA hardware are unavailable

The Phase A test verifies that the placeholder line may change while the comment text remains an exact stable anchor. The smoke-test change preserves execution on CUDA-capable machines while avoiding collection failures on CPU-only environments.

Verification

• git diff --check passed
• cargo fmt --manifest-path rust/glossapi_rs_cleaner/Cargo.toml -- --check passed
• CARGO_BUILD_JOBS=1 CARGO_INCREMENTAL=0 cargo test --manifest-path rust/glossapi_rs_cleaner/Cargo.toml passed: 326 passed, 3 ignored
• python3 -m pytest -q tests/test_pipeline_smoke.py passed with 1 skipped on this CPU-only host

This change does not alter pipeline artifact locations or public phase signatures.