feat(L3): expose renderer platform abstraction via settings.platform

[tuncayuk]

Summary

Adds a new top-level platform setting to Blits.Launch that lets apps pick
the platform abstraction layer used by the underlying Lightning renderer.

The renderer ships four implementations under @lightningjs/renderer/platforms
(WebPlatform, WebPlatformNext, WebPlatformChrome50, WebPlatformLegacy).

Until now Blits always fell back to the default WebPlatform.

Usage

Blits.Launch(App, 'app', {
  w: 1920,
  h: 1080,
  platform: 'legacy', // 'web' (default) | 'next' | 'chrome50' | 'legacy'
})

[github-actions]

Test Results: ✅ PASSED

Run at: 2026-06-09T10:39:08.984Z

Summary:
passed: 1247 failed: 0 of 1247 tests

[github-actions]

Test Results: ✅ PASSED

Run at: 2026-06-09T12:33:29.286Z

Summary:
passed: 1247 failed: 0 of 1247 tests