feat(ltdc): add LtdcFramebuffer DrawTarget and F469 framebuffer demo

[Amperstrand]

Summary

• Add a \ feature that exposes , a \ abstraction over an LTDC-backed framebuffer using .
• Provide a new \ example that initializes SDRAM, DSI, and LTDC on STM32F469I-DISCO and draws colour bars and rectangles via \ primitives.
• Complements PRs feat(dsi,ltdc): implement missing DSI commands and add DSI-compatible LTDC constructor #863 and feat(example): add NT35510 LCD support with auto-detection for F469I-DISCO #864 by demonstrating a full framebuffer-based pipeline on the NT35510/OTM8009A DSI display.

Made with Cursor

[Amperstrand]

Consolidating into #843