chore(gate): check.sh can list its lanes and run one of them

--list prints the lane names, --only "<lane>" runs that one lane, so a runner can schedule the lanes itself and a red lane can be re-run on its own. An --only run reports that lane, never the gate, and an unknown name exits 2 instead of passing with nothing run; the contract test holds both.

frontend/src/lib/gates.contract.test.ts

@@ -1,6 +1,7 @@
+import { execFileSync, spawnSync } from "node:child_process";
 import { createHash } from "node:crypto";
 import { readdirSync, readFileSync, writeFileSync } from "node:fs";
-import { relative } from "node:path";
+import { relative, resolve } from "node:path";
 import { fileURLToPath } from "node:url";
 import { ESLint, type Linter } from "eslint";
 import prettierConfig from "eslint-config-prettier";
@@ -375,6 +376,37 @@ describe("the gate snapshot (gates.snapshot.json)", () => {
   }, 120_000);
 });
 
+// Git for Windows' own bash: on a Windows PATH `bash` is often WSL's, which cannot run the script.
+function bash(): string {
+  if (process.platform !== "win32") return "bash";
+  const exec = execFileSync("git", ["--exec-path"], { encoding: "utf8" }).trim();
+  return resolve(exec, "../../../bin/bash.exe");
+}
+const checkSh = (...args: string[]) =>
+  spawnSync(bash(), ["scripts/check.sh", ...args], { cwd: REPO, encoding: "utf8" });
+
+// A runner that schedules lanes one by one learns them from `--list` and runs each with `--only`; a
+// lane missing from the list would be a lane it never runs, and an `--only` that passed with nothing
+// run would be a green about nothing.
+describe("the gate script's lane list (check.sh --list / --only)", () => {
+  it("lists every lane the script runs, in order", () => {
+    const declared = [...read("scripts/check.sh").matchAll(/^\s*lane "([^"]+)"/gm)].map(
+      (m) => m[1],
+    );
+    const listed = checkSh("--list");
+    expect(listed.status).toBe(0);
+    expect(listed.stdout.trim().split("\n")).toEqual(declared);
+    const lint = checkSh("lint", "--list").stdout.trim().split("\n");
+    expect(lint.length).toBeGreaterThan(0);
+    expect(declared.slice(0, lint.length)).toEqual(lint);
+  });
+  it("refuses an unknown lane instead of passing with nothing run", () => {
+    const r = checkSh("--only", "no such lane");
+    expect(r.status).toBe(2);
+    expect(r.stdout).not.toContain("green");
+  });
+});
+
 describe("the gate register (docs/gates.md)", () => {
   it("lists every ESLint rule narrowed, by scope", async () => {
     expect(missingFrom(eslintNarrowings(await linted()))).toEqual([]);