Add remote drag receive implementation (t=k) in dnd.c with comprehensive tests

- Add RemoteDrag state structures to state.h (RemoteDragDir, RemoteDragEntry, RemoteDrag)
- Implement drag_receive_remote_data() in dnd.c for handling t=k escape codes
- Support file transfer, symlinks, directory trees, chunked base64 data
- Add 1GB transfer cap (REMOTE_DRAG_MAX_BYTES) to prevent resource exhaustion
- Add drag_free_remote_drag() cleanup function
- Add test helpers: dnd_test_setup_remote_drag, dnd_test_get_remote_drag_temp_dir
- Add 22 comprehensive tests covering: single file, multiple files, chunked transfer,
  binary integrity, symlinks (absolute/relative), directory trees 3 levels deep,
  symlinks in directories/nested dirs, empty files/dirs, error handling,
  early termination/cancel, special characters in filenames, mixed content

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Co-authored-by: kovidgoyal <1308621+kovidgoyal@users.noreply.github.com>

Author: Copilot