Add comprehensive code quality and refactoring suggestions document

[Copilot]

Full codebase audit of Modern.Forms (221 source files, ~30k LOC) producing a numbered suggestions document with actionable refactoring items and a letter-grade scorecard.

CODE_QUALITY_SUGGESTIONS.md contains 35 numbered items (S-001 through S-035) across 10 categories:

• Testing (F): Only 12 tests exist for 221 source files. No coverage for controls, renderers, layout engines, or events.
• Error Handling (C+): 5 generic catch (Exception) sites, no diagnostics/logging on swallowed exceptions.
• Async (D+): async void in PictureBox, zero ConfigureAwait(false) usage, sync-over-async bridge in AsyncHelper.RunSync.
• API Design (B-): Mutable List<string> exposed on FileDialog.FileNames, mutable static default styles on Control.
• SOLID (C): LSP violations via 9 is ControlAdapter checks in Control.cs, static Theme/RenderManager resist extension and testing.
• Resource Management (C): SKBitmap back-buffer leaked on resize, unbounded image caches in all 3 samples, ImageCollection doesn't dispose bitmaps.
• Accessibility (F): Zero implementation — no AccessibleRole, AccessibleName, screen reader support, or mnemonic keys.
• Documentation (A- code / C+ project): XML docs are thorough, but 34 untracked TODOs and no CONTRIBUTING.md.
• Cross-Platform (B): Good WindowKit abstraction, but CI only runs tests on Windows despite building on 3 platforms.
• Code Style (A-): Consistent conventions, nullable reference types enabled, .editorconfig present.

Overall: C+. High-severity items to prioritize: S-001 (tests), S-007/S-008 (async), S-010/S-011 (mutable public API), S-021 (back-buffer leak), S-031 (accessibility).