Label index-trajectory breaks with from-to land-cover classes

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Problem

dft_rast_break() (added in #30) produces per-pixel break date and
magnitude rasters, but not a categorical "from → to" label for each break.
The optional bolt-on described in #30 would attach a land-cover transition label
to each detected break by sampling the existing IO LULC maps at break-pixel
locations on either side of break_date — turning continuous-index hotspots
into option-1-style categorical transitions (e.g. "Trees → Bare").

Why deferred from #30

This depends on the arbitrary-factor-raster generalization tracked in #19 (to
reuse dft_rast_transition() cleanly against sampled IO LULC labels), and #30
already delivers standalone value (break date + magnitude) without it.

Proposed approach

• For each pixel with a finite break_date, sample the IO LULC class in the
  year before and the year after the break.
• Reuse dft_rast_transition() to form the "from → to" label.
• Return the break raster with an added categorical transition layer (or an
  sf of break patches labelled with transitions, dovetailing with
  dft_transition_vectors()).

Depends on #19. Relates to #30.