The `Install Bun` step (oven-sh/setup-bun@v2) intermittently fails the Bun
download and reds the whole Rust Check job even when every Rust step (fmt /
build / test / clippy -D warnings) passed and windows/macos jobs are green.
Replace it with the official install script in a real 3-attempt retry loop
(no new third-party action), exporting the bin dir to GITHUB_PATH for the
JS-deps + planner-prompt drift-guard steps.
`set -o pipefail` is REQUIRED so `curl | bash` surfaces a curl download failure
(otherwise the pipeline returns bash's exit and the flake is never retried),
and an explicit ok-flag + `exit 1` after 3 attempts keeps a persistent real
failure from silently passing the step. (Codex review.)
bd4d310c95