feat: WindowChrome interface for custom title bar support

[kolkov]

Summary

Add WindowChrome interface for custom window chrome (frameless window) support. Enables replacing the OS title bar with a GPU-rendered custom title bar.

Scope

• WindowChrome interface: SetFrameless, SetHitTestCallback, Minimize, Maximize, Close, IsMaximized
• HitTestResult enum: Client, Caption, Close, Maximize, Minimize, Resize edges (N/S/E/W/corners)
• NullWindowChrome for testing

Related

• Internal: TASK-GC-001-window-chrome-interface
• Blocked by: this issue
• Blocks: gogpu frameless window, ui custom TitleBar widget

[kolkov]

Implemented in v0.11.0 (PR #13). WindowChrome interface, HitTestResult enum (13 values), HitTestCallback, NullWindowChrome — all shipped.