ci: guard-cargo runs 77 steps serially (26.2 min of x86-main 29.3) — parallel cargo-free lane in ci_guards.sh #5920

Evidence: handoff/aprender-pace-sections-1022/summary.md (8 merge-queue runs; guard-cargo median 26.2 min, step sum 2076 s vs section wall 2100 s). Countermeasure: a fail-closed parallel lane for steps listed in ci/guard-lane-steps.txt; lane steps run in private git worktrees with a cargo shim that fails the step if cargo is invoked; same SUMMARY counts, no third state. Authorised: aprender-57 relay 10-22 (stop-the-line scope).