fix: bump @b9g/platform to 0.1.19 with missing broadcast channel exports

@b9g/platform@0.1.18 was published before the BroadcastChannel feature
was added, but @b9g/platform-cloudflare@0.1.17 was published after,
causing a missing export error for setBroadcastChannelBackend.

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

Author: brainkim

packages/platform/CHANGELOG.md

@@ -0,0 +1,7 @@
+# @b9g/platform
+
+## 0.1.19
+
+### Bug Fixes
+
+- Fix missing `setBroadcastChannelBackend`, `setBroadcastChannelRelay`, `deliverBroadcastMessage`, and `ShovelBroadcastChannel` exports from `@b9g/platform/runtime`. These were added in source but not included in the 0.1.18 published build, breaking `@b9g/platform-cloudflare@0.1.17` which imports `setBroadcastChannelBackend`.

packages/platform/package.json

@@ -1,6 +1,6 @@
 {
   "name": "@b9g/platform",
-  "version": "0.1.18",
+  "version": "0.1.19",
   "description": "The portable meta-framework built on web standards.",
   "type": "module",
   "scripts": {