docs: P5 concurrency spec rewrite — one lock, one structural isolation, the invariant chain

Rewritten to the restructured model: lifetime-hardening guards (PDEATHSIG/SIGTERM/SIGALRM +
setsid/trap), per-run ABRA_DIR isolation (same-recipe runs now parallel), per-app-domain flock
(double-!testme serialisation), flock-probe janitor decision table (incl. the inode-identity
race rows), updated failure-mode table (cancel now tears down via the harness's own funnel;
reboot reaps immediately; 60-min deadline bounds everything), single-knob config table, how to
run tests/concurrency, fresh file/symbol index + deleted-symbol list for grep verification.
Also drops the last stale concurrency.limit mention from the .drone.yml header comment.

.drone.yml

@@ -35,8 +35,8 @@ steps:
 # the comment-bridge). Deploys the recipe at the PR head, runs install/upgrade/backup + any
 # recipe-local tests via the shared harness, then guarantees teardown (plan §4.2/§4.3).
 #
-# Resource safety (plan §4.2/§4.3): DRONE_RUNNER_CAPACITY=2 (nix/modules/drone-runner.nix) +
-# concurrency.limit=2 below allow two recipe runs in parallel. Concurrent-run safety is enforced by
+# Resource safety (plan §4.2/§4.3): DRONE_RUNNER_CAPACITY=2 (nix/modules/drone-runner.nix, the
+# single concurrency knob) allows two recipe runs in parallel. Concurrent-run safety is enforced by
 # the harness, not by serialisation: every run holds an exclusive flock on its app domain
 # (/run/lock/cc-ci-app-<domain>.lock) for its whole process lifetime, the run-start janitor probes
 # that lock to reap only orphans (held lock = live run, never touched), and recipe working trees