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.

frontend/vitest.config.ts

@@ -19,8 +19,8 @@ export default defineConfig({
     // collected, and the gate still prints green.
     include: ["src/**/*.test.{ts,tsx}"],
     environment: "node",
-    // E32 D4: a coverage REPORT, read and not enforced — the gate's `vitest --coverage` prints the
-    // summary; no threshold, because "every regression test seen red" is the bar, not a percentage.
+    // A coverage REPORT on demand (`npm run test -- --coverage`), never enforced and not in the gate
+    // (E32 D4, E35): "every regression test seen red" is the bar, not a percentage.
     coverage: {
       provider: "v8",
       include: ["src/**/*.{ts,tsx}"],