ci: flaky `bun install` (EEXIST link race) wedges the pre-push gate

[P4suta]

Symptom

The _playground-ensure step of just ci / just ci-parallel intermittently
fails its bun install:

bun install v1.3.14 (0d9b296a)
error: Failed to link jsesc: EEXIST
error: recipe `_playground-ensure` failed on line 1307 with exit code 1
ci-parallel: a background gate failed (see above)

Because ci-parallel is the pre-push gate (and a CI job), this aborts the
push. Re-running succeeds — it is a flaky link race in bun's installer, not
a real dependency problem. Observed repeatedly on 2026-06-17 (had to retry the
push twice while landing the CodeQL-advanced and change-aware-gating PRs).

Likely cause

bun's parallel package linker hitting EEXIST when materialising
node_modules (here jsesc) against a warm global cache — a known class of
bun flake on shared/concurrent installs.

Suggested fixes (pick one)

• Pin bun to a known-good version in mise.toml and pass
  bun install --frozen-lockfile in _playground-ensure (deterministic,
  no resolution step).
• If the race persists, fall back to bun install --backend=copyfile
  (avoids hardlink/symlink EEXIST at the cost of some speed), or retry the
  install once on EEXIST.
• Consider gating _playground-ensure out of the pre-push path (keep it in
  CI only) so a JS-install flake can't block pushing Rust changes.

Low priority / dev-experience, but it's been actively wedging pushes.