Remove the unused is_border_4() helper

CLAUDE.md

@@ -43,7 +43,7 @@
 
 C++ sources in `src/`:
 
-- `thinr_common.h` — shared inline helpers (`crossing_number`, `neighbour_count`, `is_border_4`).
+- `thinr_common.h` — shared inline helpers (`crossing_number`, `neighbour_count`).
 - `zhang_suen.cpp` — Zhang & Suen (1984).
 - `guo_hall.cpp` — Guo & Hall (1989).
 - `lee.cpp` — Lee, Kashyap & Chu (1994), 2-D adaptation.

NEWS.md

@@ -2,6 +2,12 @@
 
 * Development version bump following the 0.3.0 CRAN release.
 
+* Internal, with no change to any output: the unused `thinr::is_border_4()`
+  helper was removed from `src/thinr_common.h`. Its last callers were the
+  stentiford and pavlidis kernels, dropped 2026-05-20; nothing has referenced
+  it since. The header now declares only the two helpers that are actually
+  used.
+
 * Internal, with no change to any output: the Zhang-Suen crossing number
   `A(p)` and neighbour count `B(p)` were re-implemented inline in
   `src/zhang_suen.cpp`, `src/lee.cpp`, and `src/holt.cpp` instead of calling

src/thinr_common.h

@@ -29,12 +29,6 @@ inline int neighbour_count(int p2, int p3, int p4, int p5,
   return p2 + p3 + p4 + p5 + p6 + p7 + p8 + p9;
 }
 
-// 4-connected background test: TRUE iff at least one 4-connected
-// neighbour is background. Used to identify border pixels.
-inline bool is_border_4(int p2, int p4, int p6, int p8) {
-  return (p2 == 0) || (p4 == 0) || (p6 == 0) || (p8 == 0);
-}
-
 }  // namespace thinr
 
 #endif  // THINR_COMMON_H