docs(ci): the two gate jobs queue, they do not run in parallel

Declared side by side, so they read as parallel; the lab runs a single
act_runner with one slot, so the backend job starts at the exact second the
frontend one ends. Measured across runs #86-#91: ~4 + ~6 = ~10 minutes from the
push, not ~6.

Written down because believing the comment cost something real — the ops
release read-back budgeted the max instead of the sum and timed out on its
first live run, reporting a still-running gate as a failing one.

.gitea/workflows/check.yml

@@ -1,8 +1,14 @@
 # Server-side mirror of the local `siftlode check` gate (E1 S3 / DX-1), so `main` is always green
-# independent of any one machine. Two parallel jobs match the two local lanes:
+# independent of any one machine. Two jobs match the two local lanes:
 #   frontend — tsc (3 projects) + eslint + prettier --check + knip + vitest
 #   backend  — ruff + the full pytest suite (pure-logic + the DB-backed lane) against a real Postgres
 #
+# They are declared side by side but they DO NOT run in parallel: the lab has a single act_runner
+# with one slot, so they queue. Measured across runs #86-#91 — the backend job starts at the exact
+# second the frontend one ends, every time, and the pair costs ~4 + ~6 = **~10 minutes from the
+# push**. Anything that waits on this gate has to budget for the sum, not the max; the ops
+# `release` read-back timed out on its first real run for believing otherwise.
+#
 # Runner: the lab act_runner (LXC 115) mounts the Docker socket, so service containers work. The backend
 # runs pytest NATIVELY against a `postgres` service (not the local docker-compose.test.yml, whose
 # bind-mount is unreliable under a mounted-socket runner); fidelity is kept by installing the SAME pinned