fix(perf): enforce route budgets and ratchet coverage thresholds

Allow mode: enforce in the report config, fail the command on over-budget and
unavailable routes, and make the markdown wording follow the configured mode.
Recalibrate the budgets against the webpack build (baseline x1.15, rounded up to
the next 10 KiB) now that the old Turbopack numbers no longer apply, so the
Docker image build gate blocks genuine bundle regressions instead of only
warning. Raise the Vitest coverage thresholds to the measured floor
(37.17/30.1/31.79/38.17 -> 36/29/30/37).

scripts/performance-report.test.mjs

@@ -373,7 +373,10 @@ describe("route JS measurement", () => {
 		expect(() =>
 			validateBudgets({ ...config, routes: [budget, budget] }),
 		).toThrow();
-		expect(() => validateBudgets({ ...config, mode: "enforce" })).toThrow();
+		expect(validateBudgets({ ...config, mode: "enforce" }).mode).toBe(
+			"enforce",
+		);
+		expect(() => validateBudgets({ ...config, mode: "strict" })).toThrow();
 		expect(() => validateBudgets({ ...config, typo: true })).toThrow();
 	});
 	it("emits warnings and artifacts with exit zero when a build is unavailable", () => {
@@ -423,4 +426,51 @@ describe("route JS measurement", () => {
 		expect(markdown).toContain("not measured browser traffic");
 		expect(markdown).toContain("Raw bytes");
 	});
+	describe("enforce mode", () => {
+		const run = (dir, budgets) => {
+			const write = (relative, value) => {
+				const file = path.join(dir, relative);
+				mkdirSync(path.dirname(file), { recursive: true });
+				writeFileSync(file, JSON.stringify(value));
+			};
+			write("budgets.json", budgets);
+			return spawnSync(
+				process.execPath,
+				[
+					"scripts/performance-report.mjs",
+					"--next-dir",
+					dir,
+					"--config",
+					path.join(dir, "budgets.json"),
+					"--output-dir",
+					path.join(dir, "reports"),
+				],
+				{ encoding: "utf8" },
+			);
+		};
+		it("passes when every route is measured within budget", () => {
+			const enforce = { ...config, mode: "enforce" };
+			const runResult = run(fixture().dir, enforce);
+			expect(runResult.status).toBe(0);
+			expect(runResult.stdout).toContain("Mode: enforce");
+			expect(runResult.stderr).not.toContain("Performance budgets enforced");
+		});
+		it("fails when a route is over budget", () => {
+			const enforce = {
+				schemaVersion: 1,
+				mode: "enforce",
+				routes: [{ ...budget, maxInitialRawBytes: 1, maxInitialGzipBytes: 1 }],
+			};
+			const runResult = run(fixture().dir, enforce);
+			expect(runResult.status).toBe(1);
+			expect(runResult.stderr).toContain("Performance budgets enforced");
+			expect(runResult.stderr).toContain("/news (over-budget)");
+		});
+		it("fails when a route cannot be measured, so broken manifests do not pass quietly", () => {
+			const enforce = { ...config, mode: "enforce" };
+			const runResult = run(path.join(fixture().dir, "missing"), enforce);
+			expect(runResult.status).toBe(1);
+			expect(runResult.stderr).toContain("/news (unavailable)");
+		});
+	});
 });