Fix Wayland overlay flicker by honoring SHM buffer lifetimes

[devmobasa]

Summary

Fixes #318

• Attach the main overlay and About window buffers through SCTK’s Buffer::attach_to() so SHM slots remain active until wl_buffer.release.
• Retain a fixed-size overlay swapchain and defer rendering when every configured buffer is compositor-owned, preventing unbounded pool growth.
• Track buffer-exhaustion deferrals without counting them as presented frames.
• Scope inline-toolbar hover damage instead of falling back to full-surface repainting.
• Preserve the asymmetric file-dialog policy: entry requires a committed transparent frame, while
  restoration accepts a deferred repaint so a selected file is never discarded.

Root cause

The main overlay attached the raw wl_buffer directly to the Wayland surface. This bypassed SCTK’s slot activation.

When the inactive Buffer was dropped, its SHM memory returned to the pool’s free list before the compositor sent wl_buffer.release. With vsync disabled, the next frame could immediately repaint the same memory while the compositor was still presenting it, producing toolbar and annotation flicker.

This also meant the configured buffer count was ineffective and prevented the existing per-slot damage tracking from operating over a real swapchain.