BACKLOG — sub-phase conc

Build backlog

P1 lock-lifetime hardening: prctl PDEATHSIG + ppid race check + SIGTERM handler → teardown funnel + signal.alarm(3600) hard deadline; .drone.yml setsid/trap wrap; PEP 446 comment on lock open()
P2 flock-probe janitor: acquire_app_lock(domain) at register_run_app's call site; janitor probes per-domain lockfiles (acquired→reap under probe lock, held→leave, >120min mtime→warn); delete registry symbols
P3 per-run ABRA_DIR: /var/lib/cc-ci-runs//abra with servers+catalogue symlinks, fresh recipes/; fetch_recipe = plain clone; delete acquire_recipe_lock; route harness recipe paths through ABRA_DIR
P4 config cleanup: remove concurrency.limit from .drone.yml; maxTests is the single knob
tests/concurrency suite (19 cases, real-kernel flock, explicit invocation only)
P5 docs/concurrency.md rewrite to the new model
M1 claim (branch complete, both suites + lint green)
M2: merge to main after M1 PASS, push build green, live verification a–d

(adversary-owned)