Reuse build artifacts across CI jobs

[nathan-probert]

#98

Consolidates all C/Rust compilation into the \setup\ job. \lint, \ est, \deploy, and \dev-integration\ now reuse the uploaded artifacts instead of recompiling.

• \setup\ compiles C (\make compile_c) and Rust (\cargo build --release --target x86_64-unknown-linux-gnu) once, installs the Rust module into the venv, and uploads both artifacts.
• \lint\ no longer downloads (unused) build artifacts.
• \ est\ downloads the artifacts and installs the prebuilt Rust module instead of recompiling with maturin.
• \deploy\ downloads artifacts, sets \DEPLOY_SKIP_BUILD=1, and \deploy.sh\ skips C/Rust compilation (with a missing-artifact guard; the Docker path remains for local/dev use).
• \dev-integration\ unchanged (never compiled).
  Cuts job from ~5min -> 2:30