Tracking: run the Starling browser shell on Skyline

[codymullins]

Umbrella issue for making Skyline able to host the Starling browser shell, replacing Starling's bespoke Silk.NET plumbing (Starling.Shell.Native). Starling does not reference Skyline yet.

Good news: the present path already reconciles

Starling's GpuSurfacePresenter (Starling.Paint) builds a surface-compatible wgpu device and blends layer ops straight into the swapchain texture. Skyline's GpuContext.Create(WebGPU, surfaceFactory, …) eject + the BeginClearPass = false raw path on Skyline.Render.Frame let Starling encode its blend ops into Skyline's WindowSurface.CurrentView using Skyline's device. So present is the least of the work.

One reconciliation to confirm: device/instance ownership. Today GpuBlendEngine.CreateForSurface(window) creates its own instance + device. To run on Skyline it must instead consume Skyline's GpuContext device/queue (split out a "create with an existing device" path) so there's one device and the per-layer texture cache stays resident. Surface format and present-mode come from Skyline's WindowSurface.

The port (Starling-side work, gated by the Skyline seams below)

• Replace direct Silk.NET windowing/input/GLFW-clipboard in NativeBrowserWindow with Skyline AppWindow + FrameLoop.
• Route GpuSurfacePresenter through Skyline's GpuContext/CurrentView.
• Move the macOS bridges (MacImeBridge, MacAccessibilityBridge) behind the new Skyline seams.
• Note: samples/StarlingMock.Ios is a mock (generative-shader pages), not the real engine — there's no end-to-end engine-on-Skyline proof yet.

Blockers (parity on macOS)

• Input: carry modifier-key state on key and pointer events #5 — modifier-key state on key/pointer events
• Input: IME marked-text (preedit) composition seam #6 — IME marked-text (preedit) seam
• Accessibility: platform a11y provider seam (NSAccessibility on macOS) #7 — accessibility provider seam (NSAccessibility)

Cross-platform blockers

• Exercise & verify the GLFW + wgpu path on Windows #17 — verify GLFW + wgpu on Windows
• Exercise & verify the GLFW + wgpu path on Linux (X11/Wayland) #18 — verify GLFW + wgpu on Linux

Important for browser UX

• Input: set OS cursor shape (SetCursor) from the hit-tested element #8 — cursor shape control
• Input: pointer enter/leave window events #9 — pointer enter/leave
• Window: focus/blur (activation) events, plus occlusion signal #10 — window focus/blur (+ occlusion)
• Input/timing: wheel coalescing, normalized scroll deltas, and a vsync clock #11 — wheel coalescing / normalized scroll / vsync clock
• Window: fullscreen toggle, maximize, and a window-state-changed event #12 — fullscreen / maximize / window-state event
• Window: get/set position and bounds (save & restore window placement) #13 — window position/bounds save & restore
• Window/GPU: per-monitor DPI change and display-changed notifications #14 — per-monitor DPI / display-changed
• Native file open/save dialogs (for <input type=file> and downloads) #15 — native file dialogs
• System theme / appearance and reduced-motion change notifications #16 — system theme / reduced-motion
• CI: build + test on macOS, Windows, and Linux #19 — CI on macOS/Windows/Linux

Future / polish

• Native windowing backends for Windows and Linux (design + factory seam) #20 — native Windows/Linux backends
• Clipboard: rich formats (image + HTML/MIME) beyond plain text #21 — rich clipboard (image/HTML)
• Input: drag-and-drop (file drop into the window, and drag-out) #22 — drag-and-drop
• Interaction v2: zero-copy GPU transfer bridge (Skyline.Interaction.Gpu) #23 — zero-copy GPU transfer bridge
• Window: set window/app icon #24 — window icon
• System notifications (and optional tray icon) #25 — system notifications / tray
• GPU: wide-gamut / HDR surface color spaces (and gamma) #26 — wide-gamut / HDR color spaces
• App: single-instance + URL-scheme / deep-link handling #27 — single-instance / URL-scheme handling

Already done (no work)

Window + event loop + DPI, zero-copy wgpu present with stale-swapchain recovery, FramePacer, AppHost multi-window, native macOS chrome (Skyline.Apple), clipboard text, GPU readback, interaction/approvals tier v1.