fix: prevent platform export from leaking when user overrides module

When a user overrides a directory/cache module in shovel.json but omits
export, the platform default's named export was leaking through the
shallow merge, producing broken imports like
`import { CloudflareAssetsDirectory } from "@b9g/filesystem/node-fs"`.

Also adds a platform export compatibility test that verifies platform
packages only import symbols that @b9g/platform/runtime actually exports.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: brainkim

src/utils/config.ts

@@ -1394,6 +1394,9 @@ export function generateConfigModule(
 				const platformConfig = platformCaches[name] || {};
 				const userConfig = userCaches[name] || {};
 				const mergedConfig = {...platformConfig, ...userConfig};
+				if (userConfig.module && !userConfig.export) {
+					delete mergedConfig.export;
+				}
 				caches[name] = reifyModule(mergedConfig, "cache", name);
 			}
 			config.caches = caches;
@@ -1415,6 +1418,9 @@ export function generateConfigModule(
 				const platformConfig = platformDirectories[name] || {};
 				const userConfig = userDirectories[name] || {};
 				const mergedConfig = {...platformConfig, ...userConfig};
+				if (userConfig.module && !userConfig.export) {
+					delete mergedConfig.export;
+				}
 				directories[name] = reifyModule(mergedConfig, "directory", name);
 			}
 			config.directories = directories;

test/config-module.test.ts

@@ -418,6 +418,61 @@ describe("generateConfigModule", () => {
 			expect(module).toContain('from "@b9g/filesystem/memory"');
 			expect(module).not.toContain('from "@b9g/filesystem/node-fs"');
 		});
+
+		it("clears platform export when user overrides directory module without export", () => {
+			// Bug: user overrides module but not export → platform's export leaks through
+			const module = generateConfigModule(
+				{
+					directories: {
+						public: {
+							module: "@b9g/filesystem/node-fs",
+							path: "./public",
+						},
+					},
+				},
+				{
+					platformDefaults: {
+						directories: {
+							public: {
+								module: "@b9g/platform-cloudflare/directories",
+								export: "CloudflareAssetsDirectory",
+							},
+						},
+					},
+				},
+			);
+
+			// Should use user's module with default import, NOT platform's named export
+			expect(module).toContain('from "@b9g/filesystem/node-fs"');
+			expect(module).not.toContain("CloudflareAssetsDirectory");
+		});
+
+		it("clears platform export when user overrides cache module without export", () => {
+			// Same bug for caches: user overrides module but not export
+			const module = generateConfigModule(
+				{
+					caches: {
+						sessions: {
+							module: "@b9g/cache/memory",
+						},
+					},
+				},
+				{
+					platformDefaults: {
+						caches: {
+							sessions: {
+								module: "@b9g/platform-cloudflare/caches",
+								export: "CloudflareKVCache",
+							},
+						},
+					},
+				},
+			);
+
+			// Should use user's module with default import, NOT platform's named export
+			expect(module).toContain('from "@b9g/cache/memory"');
+			expect(module).not.toContain("CloudflareKVCache");
+		});
 	});
 
 	describe("secrets handling", () => {

test/platform-exports.test.ts

@@ -0,0 +1,110 @@
+/**
+ * Tests that platform packages only import symbols that @b9g/platform/runtime
+ * actually exports. Catches publish regressions like the missing
+ * setBroadcastChannelBackend in @b9g/platform@0.1.18.
+ */
+
+import {describe, it, expect} from "bun:test";
+import {readFileSync, existsSync} from "fs";
+import {join} from "path";
+
+const packagesDir = join(import.meta.dir, "..", "packages");
+
+/** Extract named imports from `import { A, B } from "@b9g/platform/runtime"` */
+function extractImportsFrom(
+	code: string,
+	fromModule: string,
+): Set<string> {
+	const imports = new Set<string>();
+	// Match: import { A, B, C } from "module"
+	// Handles multi-line imports
+	const escaped = fromModule.replace(/[.*+?^${}()|[\]\\]/g, "\\$&");
+	const re = new RegExp(
+		`import\\s*\\{([^}]+)\\}\\s*from\\s*["']${escaped}["']`,
+		"g",
+	);
+	for (const match of code.matchAll(re)) {
+		for (const name of match[1].split(",")) {
+			const trimmed = name.trim();
+			if (trimmed) {
+				imports.add(trimmed);
+			}
+		}
+	}
+	return imports;
+}
+
+/** Extract the final `export { ... }` block from a bundled ESM file */
+function extractExports(code: string): Set<string> {
+	const exports = new Set<string>();
+	// esbuild emits a single `export { ... }` block at the end
+	const re = /export\s*\{([^}]+)\}/g;
+	for (const match of code.matchAll(re)) {
+		for (const name of match[1].split(",")) {
+			// Handle `foo as bar` — the exported name is `bar`
+			const parts = name.trim().split(/\s+as\s+/);
+			const exported = (parts[1] || parts[0]).trim();
+			if (exported) {
+				exports.add(exported);
+			}
+		}
+	}
+	return exports;
+}
+
+describe("platform export compatibility", () => {
+	const baseRuntimePath = join(
+		packagesDir,
+		"platform",
+		"dist",
+		"src",
+		"runtime.js",
+	);
+
+	// Platform packages that have a runtime.js importing from @b9g/platform/runtime
+	const platformPackages = ["platform-cloudflare"];
+
+	it("@b9g/platform/runtime dist exists", () => {
+		expect(existsSync(baseRuntimePath)).toBe(true);
+	});
+
+	for (const pkg of platformPackages) {
+		const runtimePath = join(
+			packagesDir,
+			pkg,
+			"dist",
+			"src",
+			"runtime.js",
+		);
+
+		it(`@b9g/${pkg}/runtime dist exists`, () => {
+			expect(existsSync(runtimePath)).toBe(true);
+		});
+
+		it(`@b9g/${pkg}/runtime only imports symbols exported by @b9g/platform/runtime`, () => {
+			const baseCode = readFileSync(baseRuntimePath, "utf-8");
+			const pkgCode = readFileSync(runtimePath, "utf-8");
+
+			const baseExports = extractExports(baseCode);
+			const pkgImports = extractImportsFrom(
+				pkgCode,
+				"@b9g/platform/runtime",
+			);
+
+			expect(pkgImports.size).toBeGreaterThan(0);
+
+			const missing = new Set<string>();
+			for (const name of pkgImports) {
+				if (!baseExports.has(name)) {
+					missing.add(name);
+				}
+			}
+
+			if (missing.size > 0) {
+				throw new Error(
+					`@b9g/${pkg}/runtime imports symbols not exported by @b9g/platform/runtime: ${[...missing].join(", ")}`,
+				);
+			}
+		});
+	}
+});