OpenCV 5.1.0 picks the split harfbuzz-raster library when the system
ships it, but stores the choice only as CACHE INTERNAL. pkg_check_modules()
has already set normal HARFBUZZ_LIBRARIES / HARFBUZZ_INCLUDE_DIR variables
for plain harfbuzz, and under policy CMP0126 a normal variable shadows the
cache entry, so imgproc links -lharfbuzz without -lharfbuzz-raster and
fails on every hb_raster_* symbol. Set the normal variables as well.

--- a/cmake/OpenCVFindHarfBuzz.cmake	2026-11-06 13:34:46.897219960 +0100
+++ b/cmake/OpenCVFindHarfBuzz.cmake	2026-11-06 13:34:46.908155181 +0100
@@ -73,6 +73,10 @@
       if(HARFBUZZ_HAS_RASTER)
         set(HARFBUZZ_INCLUDE_DIR "${_hb_inc}" CACHE INTERNAL "")
         set(HARFBUZZ_LIBRARIES "${_hb_libs}" CACHE INTERNAL "")
+        # pkg_check_modules() left normal variables of these names, which
+        # shadow the cache entries under CMP0126 and drop -lharfbuzz-raster.
+        set(HARFBUZZ_INCLUDE_DIR "${_hb_inc}")
+        set(HARFBUZZ_LIBRARIES "${_hb_libs}")
         set(HAVE_HARFBUZZ 1)
       else()
         message(STATUS "HarfBuzz: found system version ${HARFBUZZ_VERSION} but it lacks the hb-raster API; building the bundled copy instead")