Speed up Docker cd equivalence CI

[plumocracy]

Summary

• feature-gate the real FastEmbed/ONNX embedder behind the default real-embedder feature
• build the Docker cd-equivalence binary with fake embeddings and no default features
• run the Docker equivalence job in parallel with the main Rust job and reduce CI random cases to 20
• cache shell init in the Docker fixture and document the new Docker test behavior

Verification

• cargo fmt
• CDS_EMBEDDER=fake cargo build --quiet --no-default-features --bin cds
• CDS_EMBEDDER=fake cargo test --no-default-features --lib --bins
• cargo clippy --all-targets --all-features -- -D warnings
• cargo test --all

Note: Docker runtime was unavailable locally, so the Docker-backed test compiled and skipped runtime execution here.