[Vela OTA] Sub-Issue 13: C# Platform strategies with AOT compliance

[JusterZhu]

Sub-Issue 13: C# Platform Strategies

Implement platform-specific update strategies in \Velaris.Sdk.

Design

• \IPlatformStrategy\ — interface with platform detection, slots, update methods
• \LinuxStrategy\ (P0) — A/B slot model, delegates to Rust FFI
• \WindowsStrategy\ (P2) — UWF/WU Agent/file overlay paths
• \StrategyResolver\ — runtime OS detection

AOT Requirements

• No reflection, no dynamic, no Moq
• All DllImport static
• Compile-time factory pattern