fix(winit): keep Linux Wayland presentation responsive

[zcourts]

Summary

• detect Linux Wayland from the active native event loop
• defer the eager startup clear to the normal first redraw on Wayland
• prefer Mailbox only when the surface advertises it, with AutoVsync as the fallback
• omit the Wayland frame-callback hint that can suppress every subsequent redraw on affected compositors, while preserving notify-before-commit on all other native targets
• leave Web and WebGPU presentation unchanged

Why

The native shell currently presents a clear frame synchronously from Event::Resumed and configures every surface with AutoVsync. On affected Linux Wayland software/composited WSI paths, that eager FIFO-backed presentation can wait in compositor dispatch and starve the same event loop needed to service it. After the first mapped frame, the Wayland frame callback installed by Window::pre_present_notify can also leave later RedrawRequested events throttled indefinitely when the callback is not delivered.

The winit 0.30.13 documentation specifies that pre_present_notify schedules this throttling callback on Wayland and is unsupported on the other native targets. The wgpu 26 present-mode documentation shows that AutoVsync selects FIFO/FIFO-relaxed, while Mailbox uses a replaceable single-frame queue without tearing. A related wgpu blocking report also records AutoVsync blocking behavior and a Mailbox workaround.

Fission already bounds redraw requests through its invalidation and frame scheduler, so this Wayland-specific callback opt-out does not introduce an idle busy loop.

Scope

Only Linux Wayland changes behavior. X11 and every other native target retain AutoVsync and notify-before-commit. Wayland surfaces without advertised Mailbox support retain AutoVsync. Web/WebGPU code paths are untouched.

Validation

• focused unit tests cover startup deferral, capability-gated Mailbox selection and fallback, Wayland callback omission, and non-Wayland notify-before-commit ordering
• native ARM64 KDE Wayland with llvmpipe/Vulkan completed repeated semantic-tree reads and redraw pumps, two screenshot captures, and three navigation loops without a stall
• the frame trace advanced through 57 redraws, the process remained live after the sequence, and captured frames were complete with no blank, torn, or partial presentation