Advertise the Rust preview platform over BLE

[OwenMcGirr]

Goal

Advertise the host platform from the Windows/macOS Rust preview through the existing BLE status characteristic.

Acceptance criteria

• Add optional platform metadata with windows or macos to the shared status JSON.
• Keep protocol version 1 and the current BLE local name.
• Use one shared serializer from both native GATT implementations.
• Do not change the shipping C# app.
• Keep frontend and native cross-platform checks passing.

Coordination

Android consumer: switchifyapp/switchify-android#2766

Merge the Android consumer first so the platform is visible as soon as the Rust preview sends it.