fix(display): prevent decoration redraw flicker during external clipboard synchronization

[AdamMagued]

Fixes #4248

Summary of Changes

1. Buffer Window Rendering State Tracking and Gating:

   • Introduced RenderState in internal/display/bufwindow.go capturing window geometry, viewport scroll coordinates, buffer modification status, cursor/selection bounds, and decoration states (active flag, dividers, scrollbar position, statusline string).
   • Added GetRenderState(), ShouldRedraw(), NeedsRedraw(), Invalidate(), LastRenderState(), and SetRedrawDebounce() to BufWindow.
   • Gated redundant redrawing in BufWindow.Display() when visual state has not changed or when events occur without buffer modifications.

2. External Clipboard Synchronization Gating and Debouncing:

   • Added duplicate write gating in internal/clipboard/clipboard.go (LastWritten cache) to prevent re-invoking external clipboard tools (such as wl-copy) when register contents are unchanged.
   • Added write debouncing support via SetWriteDebounce() and cache invalidation via ResetCache().
   • Added RedrawDebounced() in internal/screen/screen.go to coalesce rapid consecutive redraw triggers.

3. StatusLine Text Inspection:

   • Extracted FormatText() and StatusString() on StatusLine in internal/display/statusline.go to provide deterministic statusline string inspection for render state comparison.

4. Documentation:

   • Added Wayland and wl-clipboard documentation to runtime/help/copypaste.md detailing popup surface behavior under compositors without wlr-data-control, along with mitigation settings (clipboard terminal, useprimary false).

5. Automated Unit Testing:

   • Added internal/display/bufwindow_test.go verifying buffer window rendering state initialization, change detection, display gating, buffer edit tracking, external clipboard synchronization without buffer edits, and decoration change responses.
   • Added internal/clipboard/clipboard_test.go verifying duplicate write gating, debouncing, cache resets, and register tracking.