build(gate)!: coverage leaves the gate and runs on demand

Both lanes printed a coverage total with no threshold (owner decision E32
D4). No real hit as a lane: the one time the number found anything (5 bugs,
E32) it was read by hand, and since then it was a line nobody acted on. It
was also the most expensive part of the vitest lane, and the run that
flaked in E35 S1. The vitest lane runs without --coverage, the pytest lane
runs plain pytest; docs/development.md gives both on-demand commands, and
the register row says why.

backend/requirements-dev.txt

@@ -7,8 +7,8 @@
 # basedpyright: the backend type-check lane (E27 S3). Pinned like everything else here — a type
 # checker whose version floats silently changes what "clean" means, and the ratchet baseline it
 # writes is keyed to the diagnostics THIS version produces.
-# pytest-cov: the coverage REPORT the gate prints (E32 D4) — a number to read, deliberately with no
-# threshold; "every regression test seen red" is the bar, not a percentage.
+# pytest-cov: the on-demand coverage report (docs/development.md) — a number to read, deliberately
+# with no threshold; "every regression test seen red" is the bar, not a percentage.
 # pip-audit: the gate's audit of requirements.lock (scripts/check.sh), blocking at any severity.
 basedpyright==1.40.1
 pytest==8.4.2