Define the RDPBackend interface, RDPConfig, and FrameUpdate types that
abstract FreeRDP behind a pluggable backend. Add PixelBuffer for shared
RGBA frame management with partial-update support and dirty tracking.
Implement full 104-key US keyboard scancode map (JS KeyboardEvent.code
to RDP hardware scancodes) with extended-key detection helpers and
mouse event flag constants matching MS-RDPBCGR.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
