feat: Platform detection and conditional test skipping

[brainkim]

Summary

When writing cross-platform tests, we need the ability to:

1. Detect the current platform at runtime
2. Conditionally skip tests based on platform

Use Case

ZenDB has tests that use platform-specific features:

• Some tests use require() which doesn't work in browsers
• Some tests need database drivers only available in Node/Bun
• Browser-specific tests (OPFS driver) should only run in browser

Proposed API

import { test, describe, expect, platform, skipIf } from "@b9g/libuild/test";

// Option 1: Platform detection constant
platform; // "bun" | "node" | "chromium" | "firefox" | "webkit"

// Option 2: Platform predicates
import { isBun, isNode, isBrowser } from "@b9g/libuild/test";

// Option 3: skipIf helper
test.skipIf(isBrowser)("uses require()", () => {
  const { NOW } = require("../src/database.js");
  // ...
});

// Option 4: test.skip/describe.skip (already in bun:test and node:test)
if (isBrowser) {
  test.skip("uses require()", () => {});
} else {
  test("uses require()", () => {
    // ...
  });
}

Current Workaround

None - tests either fail on incompatible platforms or we maintain separate test files.

Additional Context

Both bun:test and node:test support test.skip and describe.skip. The browser shim would need this added.

For platform detection, the test runner already knows the platform - it just needs to expose it to the test code (perhaps via define in esbuild).

[brainkim]

Additional Issue: Platform-specific imports

When running libuild test -p bun, esbuild bundles the test files and their dependencies. If any dependency imports from "bun" (not "bun:test"), it fails:

✘ [ERROR] Could not resolve "bun"

    src/bun.ts:8:18:
      8 │ import {SQL} from "bun";

The test runner currently externalizes bun:test:

external: platform === "bun" ? ["bun:test"] : [],

But it should also externalize "bun" and other platform-specific modules:

external: platform === "bun" ? ["bun:test", "bun", "bun:*"] : [],

This is blocking adoption of the cross-platform test runner for projects that have platform-specific driver code.