Add dft_transition_vectors() for exploded change patches

[NewGraphEnvironment]

Summary

• New exported function dft_transition_vectors() vectorizes a transition SpatRaster into sf polygons — one row per connected patch of pixels sharing the same transition type
• Supports optional zones parameter for spatial attribution (sub-basins, parcels, management units — any sf polygon set)
• Supports patch_area_min for filtering small patches before returning
• Returns patch_id, transition, area_ha, and optional zone column
• 13 new tests covering structure, area consistency, zone attribution, filtering, and input validation

Fixes #18
Relates to NewGraphEnvironment/sred-2025-2026#16

Test plan

• devtools::test() — 179 pass, 0 fail (13 new)
• Total vector area matches raster summary area
• Patch IDs are unique, transition labels valid
• Zone attribution and patch_area_min filtering work
• Input validation: non-SpatRaster, non-factor, geographic CRS, missing zone_col

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