web(i18n): one-way ceiling on isZh branching (#5805)

The isZh migration was losing ground (12 -> 31 files over 90 days at
filing; 28 today outside web/lib/i18n). Count the files that still
branch on isZh outside the migration home and fail web CI if the number
grows. Migrating branches lowers the count; then ratchet the CEILING.

Signed-off-by: CodeWhale Bot <bot@codewhale.net>
Co-authored-by: CodeWhale Bot <bot@codewhale.net>

web/lib/i18n/iszh-ceiling.test.ts

@@ -0,0 +1,38 @@
+import { describe, expect, it } from "vitest";
+import { readdirSync, readFileSync, statSync } from "node:fs";
+import { join, relative } from "node:path";
+
+const ROOT = process.cwd();
+const MIGRATION_HOME = join("lib", "i18n");
+// One-way ceiling (#5519): the isZh migration converges or holds, never
+// regresses. To lower the number: migrate branches into lib/i18n, then set
+// CEILING to the new count reported by this test.
+const CEILING = 28;
+
+function walk(dir: string, out: string[] = []): string[] {
+  for (const entry of readdirSync(dir)) {
+    if (entry === "node_modules" || entry === ".next" || entry === ".git") continue;
+    const full = join(dir, entry);
+    if (statSync(full).isDirectory()) walk(full, out);
+    else if (/\.[jt]sx?$/.test(entry)) out.push(full);
+  }
+  return out;
+}
+
+describe("isZh one-way ceiling (#5519)", () => {
+  it(`keeps isZh branching outside ${MIGRATION_HOME} at <= ${CEILING} files`, () => {
+    const offenders = walk(ROOT)
+      .map((file) => relative(ROOT, file))
+      .filter(
+        (file) =>
+          !file.startsWith(MIGRATION_HOME) &&
+          !/\.test\.[jt]sx?$/.test(file) &&
+          readFileSync(join(ROOT, file), "utf8").includes("isZh"),
+      )
+      .sort();
+    expect(
+      offenders.length,
+      `isZh branching outside ${MIGRATION_HOME} (migrate, then lower the CEILING):\n${offenders.join("\n")}`,
+    ).toBeLessThanOrEqual(CEILING);
+  });
+});