refactor(i18n): ask the catalogue for a sentence in one helper

`t()` answers a missing key with the key, so "does this code have a
sentence" was a comparison written inline at every site that wanted a
code's sentence or a fallback, twenty-odd times in several spellings.
translateOrNull, translateCodeOrNull and translateCode in
src/lib/i18n/translate-code.ts now hold that probe, every site uses
them, and a guard refuses the inline comparison anywhere else.

src/__tests__/translate-probe-guard.test.ts

@@ -0,0 +1,96 @@
+/**
+ * Whether the catalogue has a sentence is asked in one place.
+ *
+ * `t()` answers a missing key with the key itself, so "does this code have a
+ * sentence" is a comparison of the answer with the question. Written inline it
+ * was copied once per surface that wanted a code's sentence or else a
+ * fallback. It lives in `src/lib/i18n/translate-code.ts` now
+ * (`translateOrNull`, `translateCode`), and this guard refuses the comparison
+ * anywhere else under `src/`.
+ *
+ * Two shapes are read: a translation held in a variable and compared with the
+ * key it was asked for (`const v = t(key); v === key`), and the inline form
+ * (`t(k) !== k`). The helper itself must match, so a matcher that stops
+ * finding the shape fails instead of passing over nothing.
+ */
+import { readdirSync, readFileSync } from "node:fs";
+import { join, relative } from "node:path";
+import { describe, expect, it } from "vitest";
+
+const SRC = join(__dirname, "..");
+const HELPER = "lib/i18n/translate-code.ts";
+
+function walk(dir: string, out: string[] = []): string[] {
+  for (const entry of readdirSync(dir, { withFileTypes: true })) {
+    if (entry.name === "__tests__" || entry.name === "generated") continue;
+    const path = join(dir, entry.name);
+    if (entry.isDirectory()) walk(path, out);
+    else if (/\.tsx?$/.test(entry.name) && !entry.name.includes(".test."))
+      out.push(path);
+  }
+  return out;
+}
+
+function escape(text: string): string {
+  return text.replace(/[.*+?^${}()|[\]\\]/g, "\\$&");
+}
+
+/** The probes in one source text, one line each. */
+function probes(source: string): string[] {
+  const found: string[] = [];
+  // Inline: `t(K) !== K` / `t(K, p) === K`, K an identifier or a template.
+  const inline =
+    /\bt\(\s*(`[^`]*`|[A-Za-z_$][\w$]*)\s*(?:,[^()]*)?\)\s*[!=]==\s*(`[^`]*`|[A-Za-z_$][\w$]*)/g;
+  for (const m of source.matchAll(inline)) {
+    if (m[1] === m[2]) found.push(m[0]);
+  }
+  // Held: `const v = t(K…)` (or `K ? t(K) : null`) and later `v === K` or
+  // `K !== v`.
+  const held =
+    /\b(?:const|let)\s+([A-Za-z_$][\w$]*)\s*=\s*(?:[\w$.]+\s*\?\s*)?t\(\s*([A-Za-z_$][\w$]*)\b/g;
+  for (const m of source.matchAll(held)) {
+    const [, value, key] = m;
+    const compare = new RegExp(
+      `\\b${escape(value)}\\s*[!=]==\\s*${escape(key)}\\b|\\b${escape(key)}\\s*[!=]==\\s*${escape(value)}\\b`,
+    );
+    if (compare.test(source.slice(m.index))) found.push(m[0]);
+  }
+  return found;
+}
+
+const files = walk(SRC).map((file) => ({
+  file: relative(SRC, file),
+  source: readFileSync(file, "utf8"),
+}));
+
+describe("the catalogue probe lives in one helper", () => {
+  it("recognises both shapes", () => {
+    expect(probes("const s = t(key);\nreturn s === key ? a : s;")).toHaveLength(
+      1,
+    );
+    expect(probes("if (t(`x.${c}`) !== `x.${c}`) go();")).toHaveLength(1);
+    expect(
+      probes("const label = t(key, params); if (key !== label) {}"),
+    ).toHaveLength(1);
+    expect(
+      probes("const v = key ? t(key) : null; return v && v !== key ? v : f;"),
+    ).toHaveLength(1);
+    expect(probes("const s = t(key); return s;")).toEqual([]);
+    expect(probes("if (row.key === key) {}")).toEqual([]);
+  });
+
+  it("finds the probe in the helper", () => {
+    const helper = files.find(({ file }) => file === HELPER);
+    expect(helper).toBeDefined();
+    expect(probes(helper!.source).length).toBeGreaterThan(0);
+  });
+
+  it("finds it nowhere else", () => {
+    const offenders = files
+      .filter(({ file }) => file !== HELPER)
+      .flatMap(({ file, source }) =>
+        probes(source).map((p) => `${file}: ${p}`),
+      );
+    expect(offenders).toEqual([]);
+  });
+});