Record is_border_4() as deliberately retained, not orphaned

CLAUDE.md

@@ -44,6 +44,10 @@
 C++ sources in `src/`:
 
 - `thinr_common.h` — shared inline helpers (`crossing_number`, `neighbour_count`, `is_border_4`).
+  `is_border_4` has **no caller today** and is retained deliberately as part of
+  the shared 8-neighbour vocabulary a new kernel would be written against (its
+  last callers, `stentiford` / `pavlidis`, were dropped 2026-05-20). See the
+  comment on its definition; do not "clean it up" without reading that first.
 - `zhang_suen.cpp` — Zhang & Suen (1984).
 - `guo_hall.cpp` — Guo & Hall (1989).
 - `lee.cpp` — Lee, Kashyap & Chu (1994), 2-D adaptation.

src/thinr_common.h

@@ -30,7 +30,21 @@ inline int neighbour_count(int p2, int p3, int p4, int p5,
 }
 
 // 4-connected background test: TRUE iff at least one 4-connected
-// neighbour is background. Used to identify border pixels.
+// neighbour is background. Identifies border pixels.
+//
+// NO CALLER TODAY -- retained deliberately, not orphaned. Every kernel
+// currently in src/ tests deletability through crossing_number() /
+// neighbour_count() instead; this predicate was last used by the
+// `stentiford` and `pavlidis` kernels, dropped 2026-05-20. It is kept
+// because it is part of this header's reason to exist -- the shared
+// 8-neighbour vocabulary a NEW thinning kernel is written against --
+// and an uncalled `inline` emits no code, so it costs nothing at
+// runtime or in binary size.
+//
+// The 2026-07-28 review left "delete it or state that it is deliberate"
+// open as a judgement call; this comment is that decision. If a future
+// reader finds this helper still callerless and the header's shared-
+// vocabulary role no longer holds, deleting it is safe and expected.
 inline bool is_border_4(int p2, int p4, int p6, int p8) {
   return (p2 == 0) || (p4 == 0) || (p6 == 0) || (p8 == 0);
 }