Return removed patches raster from dft_rast_transition() #17

Problem

When using patch_area_min in dft_rast_transition(), there's no easy way to see what was removed. Users need to visually inspect filtered patches to decide if their threshold is too aggressive or too lenient — especially when ground-truthing against satellite basemaps.

Proposed Solution

Add a $removed element to the return list of dft_rast_transition():

result <- dft_rast_transition(classified, from = "2017", to = "2023",
                              patch_area_min = 5000)

terra::plot(result$raster)   # transitions that survived filtering
terra::plot(result$removed)  # transitions that were cut

• When patch_area_min = NULL (default): $removed is NULL, no change to current behavior
• When filtering is active: $removed is a factor SpatRaster with the same levels/colors as $raster, containing only the small patches that were masked
• Plottable, overlayable, passable to dft_map_interactive()

This lets users iterate on their threshold visually without leaving R.

Relates to #14