feat: implement X11 and Cocoa drop-targets, popups, and selection gaps

Resolve gaps identified in yasl-gaps-report.md:
- X11:
  - Add XDND v5 file drop support and text/uri-list parsing (YASL_INPUT_DROP_FILES)
  - Dispatch surface drop-target events (YASL_DRAG_OVER, YASL_DRAG_LEAVE, YASL_DRAG_DROP, YASL_DROP_OVER, YASL_DROP_LEAVE)
  - Implement override-redirect popups (surface_create_popup, surface_popup_reposition)
  - Support drag images with override-redirect icon window
  - Set YASL_CAP_POPUPS capability
  - Handle XIHierarchyChanged reset cleanly

- Cocoa:
  - Dispatch surface drop-target events for internal drags and external NSDraggingDestination
  - Implement child window popups (cocoa_surface_create_popup, cocoa_surface_popup_reposition)
  - Implement primary selection via dedicated pasteboard
  - Support drag images with floating borderless NSWindow
  - Track pointer buttons and push YASL_INPUT_RESET on focus loss
  - Set YASL_CAP_POPUPS and YASL_CAP_PRIMARY_SELECTION capabilities

- Tests:
  - Add comprehensive unit test suite in tests/x11/test_tablet.c

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