perf(i18n): stop serializing the full catalogue on public pages

The root layout fed the entire ~254 KB message catalogue to
NextIntlClientProvider, which serializes it into the RSC payload of every
page. A public page only reads ~59 KB of it; the ~195 KB difference is staff
tool copy that only /admin, /admin-next and /mod ever render.

The root provider now installs the reduced catalogue and each staff route
group re-installs the full one, since a nested NextIntlClientProvider replaces
the parent message set rather than merging into it. pages.admin.actions stays
public because the shared useServerAction hook runs on public pages too.

Measured: 253 KB -> 59 KB of messages per public page (raw JSON).

Guard tests cover all three halves: the root keeps using the reduced
catalogue, every staff layout installs its own full provider, and no public
tree reads a staff-only namespace.

src/lib/i18n-client-messages.test.ts

@@ -85,7 +85,12 @@ describe("publicClientMessages", () => {
 			if (key === "admin") continue;
 			expect(result[key]).toBeDefined();
 		}
-		expect(result.pages.admin).toBeUndefined();
+		// `pages.admin` survives only in the shape the shared server-action hook
+		// needs: the generic `actions` copy, nothing staff-specific.
+		expect(Object.keys(result.pages.admin)).toEqual(["actions"]);
+		expect(result.pages.admin.actions.saved).toBe(en.pages.admin.actions.saved);
+		expect(result.pages.admin.nav).toBeUndefined();
+		expect(result.pages.admin.catalog).toBeUndefined();
 	});
 
 	it("actually shrinks the catalogue", async () => {
@@ -111,3 +116,59 @@ describe("publicClientMessages", () => {
 		expect(en.pages.admin).toBeDefined();
 	});
 });
+
+/**
+ * Scoping only works if the reduced catalogue goes to the client *and* every
+ * staff route group re-installs the full one. These two halves drift silently:
+ * narrowing the root without adding a staff provider renders raw keys in the
+ * admin panel, and the reverse just costs bytes. Both are asserted here.
+ */
+describe("client catalogue scoping is wired up", () => {
+	const STAFF_LAYOUTS = [
+		"src/app/admin/layout.tsx",
+		"src/app/admin-next/layout.tsx",
+		"src/app/mod/layout.tsx",
+	];
+
+	it("the root layout feeds the reduced catalogue to its provider", () => {
+		const src = readFileSync(join(process.cwd(), "src/app/layout.tsx"), "utf8");
+		expect(src).toContain("publicClientMessages(await getMessages())");
+	});
+
+	for (const layout of STAFF_LAYOUTS) {
+		it(`${layout} re-installs the full catalogue`, () => {
+			const src = readFileSync(join(process.cwd(), layout), "utf8");
+			expect(src).toContain("NextIntlClientProvider");
+			expect(src).toContain("getMessages()");
+		});
+	}
+});
+
+/**
+ * A client component that reads `admin.*` / `pages.admin.*` outside a staff
+ * route group would fall back to raw keys once the root catalogue is scoped,
+ * because the provider it inherits no longer carries them. Staff trees are the
+ * ones that install the full catalogue; the allowlist holds shared code that
+ * deliberately runs on public pages too.
+ */
+describe("staff namespace consumers sit in a staff tree", () => {
+	const STAFF_TREE =
+		/^src[\\/](app[\\/](admin|admin-next|mod)[\\/]|components[\\/]admin[\\/]|features[\\/](catalog|operations|history|housekeeping)[\\/])/;
+	const ALLOWLIST = new Set(["src/hooks/use-server-action.ts"]);
+	const CLIENT_STAFF_NS = /useTranslations\((["`])((admin)|(pages\.admin))/;
+
+	it("there is source to scan", () => {
+		expect(sourceFiles(join(process.cwd(), "src")).length).toBeGreaterThan(500);
+	});
+
+	it("no stray consumer of a staff namespace", () => {
+		const offenders: string[] = [];
+		for (const file of sourceFiles(join(process.cwd(), "src"))) {
+			const rel = file.slice(process.cwd().length + 1).replaceAll("\\", "/");
+			if (rel.includes(".test.")) continue;
+			if (STAFF_TREE.test(rel) || ALLOWLIST.has(rel)) continue;
+			if (CLIENT_STAFF_NS.test(readFileSync(file, "utf8"))) offenders.push(rel);
+		}
+		expect(offenders).toEqual([]);
+	});
+});